This is for nobodies and maybe C-list celebs that will no longer pass the first filter. "Damn, more than 200 applicants. Throw out anyone over 40!". When this happens it does not matter how much plastic surgery you have had to look younger.
I guess this is a bit like applying for tech jobs in some places. It does not matter that you have all the qualifications. Too old and your out before the process even started.
Also, hiring adults to play teens is a very specific case which has the advantage of avoiding child labor laws.