According to [Uncanny] X-Men #1, Bobby Drake was sixteen when the team made its public debut. Furthermore he describes himself as a "couple" of years younger than his friends. We also know that Hank is somewhat older than Scott, Warren, and Jean. So, the short answer to your first question is: Bobby is 16, Warren, Scott, and Jean are 18, and Hank is probably 18 or 19.
It's not specifically clear whether Bobby is literally two years younger, or if "couple" is being used in a more figurative sense. I think it's safe to take at face value though, given that he is clearly presented as a member of their age cohort (along with Lorna and Alex); is meant to be reasonably close to the same age as Peter Parker and Johnny Storm (who should also be in the same general range as the original X-Men, as they were all teenagers at the same time); and lastly when Claremont revived the X-Men publication years later, Bobby was depicted as now comfortably a young adult, finishing up his accounting degree, at the same time that Jean was dealing with her first job drama, and Warren was off with the family business.
I also think it's safe to guess Hank is no more than a year older than the others. He's treated as a peer of his teammates rather than as a helper to the Professor, and seems comfortable as a member of the group even outside of X-Men training. Hank's more advanced academic pursuits with respect to his teammates are also shown to be a product of his intelligence and not simply him being in a more senior class, which I think furthermore point to him being loosely their age.
Later authors have occasionally given different ages to the first team, or at least to Scott. Bendis, who wrote them for the longest, identifies the younger Scott as sixteen when he first comes face to face with his modern counterpart (Uncanny X-Men volume 3 no. 12). In Matt Fraction's inexplicably titled Dark Avengers/Uncanny X-Men: Exodus, Scott gives a televised speech in which he claims to have joined the X-Men at age 15. And in Stuart Moore's X-Men Origins Cyclops, Scott is shown to have first met Charles when he was 17 (which works), but he also dates X-Men #1 as taking place when Scott was 21 (which contradicts every other version of the story).
I think it makes the most sense to just go with the original story, and take 18 as the team's median age. I think you could also just as well argue that the 15-17 number isn't in contradiction per se, but only reflects Scott's age when Charles started training him, or even when he first became Cyclops, and not his age in X-Men #1 when Jean joined the team and they confronted Magneto for the first time.
I know some fans also feel that the Bendis' run discounts the possibility that they're in their late teens, because he writes them so young, but I think that's kind of ridiculous. Bendis writes everyone like that, and in any case people are basically the same at 14 as they are at 34, so how much more mature does anyone get from 16 to 18, really?
I think having them remain (mostly) at 18+ is the most plausible reading of the original run, and the most consistent with their general dislike of Charles recruiting actual child soldiers in later classes, and as well with Marvel's editorial policy that the X-Men have existed for ten years in-universe in perpetuity and are now just shy of 30.
Second question: Their ages later on, when the All-New, All-Different team first formed aren't given directly, but they're easy to work out.
So, at time of first death, Jean was 24ish depending on when her birthday falls in the year. Scott and Warren are the same age, Bobby is two years younger, and Hank is probably around 1 year older.
How much time passes between Jean's introduction to Storm, Thunderbird, Wolverine, Banshee, etc and her death isn't specified, and during his first run, Claremont tried to maintain a pretense of everything occurring in real time, but already it had started to become untenable and can't be trusted.
At the time of Jean's death, the core Claremont trio of Storm, Wolverine, and Nightcrawler are still very early in getting to know each other as people, despite spending a lot of time together on page. So I think it's very likely that only a few months have passed between the new recruits joining the team and when Jean died and the remaining original X-Men make their departure. So I would just treat the Dark Phoenix Saga as taking place in the same year as the first mission to Krakoa, with Jean/Scott/Warren remaining 24 throughout.
Third question: To get to their ages at M-Day, I need to do something cataclysmically stupid. Something that I always tell everyone to never do when talking about the X-Men: I have to talk about how old Kitty Pryde was. She who has had three separate, non consecutive, fifteenth birthdays; and she who, famously, began the 1990s three years older than Jubilee, but still managed to end the decade two years younger than her. But, for once, bringing her into things is going to make everything else make more sense.
In X-Treme X-Men we're told directly that Kitty joined the X-Men five years ago. This is useful to know, because she joined the team on the exact same day that Jean died, so it has also been 5 years since that happened. X-Treme X-Men takes place at the same time as New Mutants volume 2, later to be relaunched as New X-Men: Academy X, which documents the first academic year of the Xavier School post its outing as a mutant boarding school. The two books ran parallel to one another and reference the same events in the lives of the original New Mutants.
Academy X concludes during the House of M crossover itself, and with the depiction of the typical US/Canadian school dances, and the inclusion of a year book issue, I think it's reasonable to conclude that the entire Academy X era takes place over about one full school year. So adding that together, M-Day should fall around 6 years after Jean's first death, making her, Scott, and Warren around 29-30, Hank 30-31, and Bobby 27-28.
If you want corroboration while also having the chance to experience a desperate urge to kick something, here's a cropped out section of the House of X/Powers of X timeline.
My version has a type mistake, where years 49 and 50 were incorrectly reversed. Genosha should be year 50, and Moira's "death" at 49. (IIRC later print editions fixed this) M-Day and Decimation would happen in year 51. The Year 47 event references the storyline beginning in Jim Lee's X-Men #1, and so the formation of the Claremont X-Men would likely be in Year 45 or 46. Scott and co being 30ish at Year 52, and working backwards...It actually all kind of lines up?
Which I hate! I hate that it works. It forces you to accept that everything after M-Day occurs over the course of just one single year. And it's so annoying that it's actually kind of possible, when you remember how much everyone loved decompressed storytelling in those days, and subsequently how almost nothing actually happened for a decade and a half of comics. But still! That's so ridiculous! Why do they do this!
Hope that helps! Thanks for the ask.
