I'm not sure but I've read here that it gets better with age if you put in the work

I haven't found that to be the case at all. Even the women looking for commitment know they aren't going to find a man who will wait until marriage, and they don't want to either.

as you get older your value should increase if you work on it

Guys' SMV tops around 36 or so. Obviously varies a bit

for example the youngest girl a 40 year old would be with is 25 say for example.

Doesnt really work this way. The amount of girls that will be comfortable/interested in fucking you diminishes as the age gap grows, but there is quite a lot of young girls who even actively go for older men. In practice all that happens is that you need to vet the girls a little bit more to figure out if she is into older men, and then sexualizing quickly if she is; simply to not fall into the "cool uncle"-category (or creepy uncle if you fail lol).

I.e. you can still fuck 20 year olds at 50. They're just gonna be harder to find than when you are younger. Read Uncle Vaz' posts on reddit TRP for more on this.

now (when they hit "the wall") they are less carefree to have sex and will put their pussy on lock

First off; if attracted enough they will fuck you anyway. But it's true that women change when they hit the wall. So even as an older guy, it can be easier to pull a girl who's 23 than a girl who's 30 with baby rabies.

what’s the red pill method to counter this ?

You dont really need to counter anything. Work on your SMV i.e. Game, Looks, Money, Status and reap the benefits. Adjust your game to fit the age group you're aiming for, whatever your age.

I think a man's peak years are age 35-45. Then you start getting gray, lower testosterone, wrinkles, etc. I'm pushing 47 and I won't even consider dating or spending time with any women other than my wife. Sex is just a transaction these days. I do get a lot of single moms checking me out when I sport my gray beard and the 20-somethings in the West ignore me but like that other guy on here I can pull young pussy all day in SE Asia.

. But the age group of women you go for may also increase, for example the youngest girl a 40 year old would be with is 25 say for example.

Why??

You can date 25 yr old women at any age.

My question is at that age girls start to go past their prime, and now they are less carefree to have sex and will put their pussy on lock to find a relationship and commitement

Well, they try. It doesn't work so great, and women are women - they still love sex.

So is this theory true, and if so what’s the red pill method to counter this ?

Stay hot and date younger women.

If you like older women you just need to make it clear that you're fine with them, attracted to them, but you won't be fucking around: they can take or leave it.

If you're hot, they'll take it.