Ok, but based on the scene in ANXM #1, when Beast come get the O5 in the past, it's at least one year after Jean joined the X-Men... So at least 16...
UXM1 #8
uxm1-8.jpg
ALXM #1
anxm-1.jpg
Also look at the letter O5 Scott is writing to Jeen : no way it happen a couple of weeks after she joined the X-Men...
I don't think the author tried very hard to get things rights... lol