I was thinking about this the other day, and I really think that, bar Rogue due to all the crap surrounding her, you can narrow down at least something of a bracket.
For starters, it's outright stated that Scott is 16 when Xavier finds him, so my feeling on it is something similar to this:
Original five fighting Magneto:
Cyclops - 16
Beast - I would guess 16 but he MIGHT be 17
Angel - Around 16
Jean - 16 to be the same age as Cyclops
Bobby - 14
Mimic - Not sure
Changeling - Not sure
Havok - should be about 2-4 years younger than Cyclops due to their origin making Cyclops old enough to actually look after him.
Polaris - Don't know.
Deadly Genesis people, don't care. my guess though, is that Krakoa happens about... 4 years after the start of the series, so for example, if Cyclops is 20 by this point:
Colossus - 16 (seems the best age for him to stay in keeping with Claremont's intentions)
Storm - 16-18?
Logan - I dunno, 105?
Banshee - He's easily in his 20s if not 30s. He seems even older than Cyclops iirc.
Thunderbird - Don't know.
Sunspot - Late teens early 20s?
Nightcrawler - Mid to late teens.
A year or so later, they bring in Shadowcat who iirc is 13, which would make:
Cyclops & Jean 21 (as we see them going to bars etc)
Colossus 17 so there's a 4 year gap between him and Katya.
I may be remembering wrong, so please, any corrections are welcome.
Yeah, iirc, I just couldn't nail it down. I also considered that both he and Charles seemed to be in love with Moira, and if they are rivals for her attention, then Sean could have even been in his 40s.
That was when Xavier was what, evil? Or an imposter? Or undercover?
The scene still works even if we exempt Colossus. Because of his powerset he didn't have to worry about being hurt, especially considering how Claremont (in a nice way) wouldn't shut the **** up about how durable he was.
Nightcrawler was around 20 when he was recruited (he has his 21st birthday during the annual with Doctor Strange), and Storm could easily be 20 when she's recruited, unless I'm remembering wrong.
Probably of that time when the dark side of his psyche took control during his dream? The evil Xavier with a cape thing (green in color, naturally, as all villains in Marvel are green). Appeared in TAS, too.
Way made him responsible for Wolverine's brainwashing