Maybe it's just me

So there is a movie coming out called 17 Again. The basic plot is that Matthew Perry magically gets turned into his 17 year old self in order to relive his high school days. The younger version of Matthew Perry is played by the hunky Zac Efron. I want to know why Matthew Perry is the older Zac Efron and why in the world they wouldn't have casted Rob Lowe. Rob Lowe and Zac Efron are practically identical except that Zac looks about 20 years younger.