He was old enough as he is the same age as Rand. Egwene just got permission to braid her hair which meant the "courtship" between her and rand could officially take place now that she was deemed mature enough to marry. She was 17 though it can happen as early as 14. It all depends on when the circle deems the girls mature enough. Perrin is 19/20 at the start of the books.
It wasn't really a nonsensical change since Perrin himself even says he would likely have married/courted Laila if he stayed in the Two Rivers a few more years. It was also made to give him some character since you don't get much early on in the books and a lot of it is internal which doesn't work well for TV/Film. If you care you can read the "official" explanation at https://winteriscoming.net/2021/11/2...change-perrin/
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He is in Season 2. https://winteriscoming.net/2022/02/2...time-season-2/