Because I notice Alex Summers is in this movie, and if he's a teenager while Professor X and Magneto are in their 20s, that would make him at least 20-40 years older than his older brother Scott (Cyclops). Explanation?
It is possible. My own father had a sister who was 16 when Dad was born. Being 20 years older than your sibling is not common but does happen sometimes. If a woman has her first kid at 16 she could have another one when she's 40. Many women are perfectly fertile at age 40.
Also the movie did somewhat change the continuity from the other films; which isn't too surprising. Comic books are known for constantly re-writing their own backstories & continuity.
******SPOILERS AHEAD******
In the prologue to "X-Men 3" and a cameo in "Wolverine" Prof X is shown as a middle aged man who's not crippled (and played by Patrick Stewart who's been made to look younger through CGI) but in this film he gets crippled while still in his 20s and sporting a full head of hair.
From a nudist perspective there is a bit of a connection to the previous films. In the comics Mystique wears a long white dress but in the previous films she prefers to go nude when she's in her natural blue state. It was decided while working on the first movie that Mystique would be nude because wearing actual clothing would be a liability for a shape shifter. She'd have a harder time quickly mimicking a soldier during a break in or an escape if she's wearing a real white does. For much of this movie she mimics the appearance of a blond white girl and Erik asks her "if you're using half of your concentration to look normal than you're only half paying attention what whatever else you're doing. I'm just pointing out something that could save your life." A bit later she returns to her blue scale covered form and walks around the mansion nude. Charles is shocked and asks "where are your clothes?" and she just says something to the effect of "I'm just being me and not hiding who I am."
I was a bit let down by the ending, I felt that the one they had could have better fit the continuity of the other X-men films. Admittedly I had totally forgotten that Prof. X was not disabled in the prologue to X-Men 3. My own opinion of how the ending could have worked would have involved most of the stuff that we got, with a few changes. I liked Charles and Erik's fight on the beach and Erik accidentally deflecting the bullet that crippled Charles. But in the ending that we got Erik suddenly turns from a mostly sympathetic guy with a bit of an edge into a supervillain almost at once. I'd have liked to see a reconciliation between Charles and Erik.
Erik was fairly justified in killing Shaw, against Charles' wishes; the guy did murder Erik's mother in front of him. You can even understand him wanting to turn the missiles back on the military guys who fired them; "I've been at the mercy of men who were just following orders before." Crippling Charles was an accident during the fight. I'd have liked it to end with Charles begging his friend to give the idea that humans and mutants can peacefully coexist another chance and Erik (racked with guilt over having accidentally crippled his best friend) reluctantly agreeing but perhaps saying that he still believes that Charles is hopelessly naive but will try to play it his way.
I just think that the character would be more interesting that way. The producers are hoping for a new trilogy with this film and I think that it would be more interesting to watch Erik slowly become evil, rather than just end the film with BAM - he's a supervllain!!!!!!!!