So i wanted to know if leopard geckos can go infertile by the age of 5
I am not an expert in Leos but I would definitely trust @westridge with questions about them. But I will say that sounds young, maybe the morph has known fertility issues. What is/are the morph(s)?
First of all is it a male or female, also do they seem healthy. I would say that a gecko at that age would still be fertile but it depends on the previous questions and it might make a difference what morph it is.
Okay then, so what depends on the answer to your main question is if it’s male or female and if the leo is in good health.
Female and in good condition
Can we get some pictures of the gecko in question? And have you taken her to the vet for a check up at all? Would be good to know her diet too.
She should be fertile, when you breed her, pair the male with her once you see eggs forming in her. I’d recommend only breeding her if she was bought from a reputable breeder (not big chain pet store, rescue, shady/backyard breeder, etc.). If you’re breeding it wouldn’t be a bad idea to take her to a vet and make sure that she’s totally healthy (no MBD, strong muscles, etc.). Do you know everything about the actual breeding process? Supplements, protein, low chitin, incompatible morphs(doesn’t apply with a normal), etc.
The short answer is that, yes, of course it’s possible to have infertile animals at a young age. There are a zillion things that can cause it (obesity, NDBE, inbreeding, etc.).
But, no, it is not normal, and if you suspect that infertility is the case you need to take your geckos to a qualified herp vet. There they can examine your geckos and discuss husbandry practices.
You should not assume it’s the female unless you’ve previously & recently confirmed that the male is fertile.
Dusted meal worms and crickets