at how old do female bettas start to lay eggs?

Answer 1

6-8 months is about when you see them getting into breeding condition. This can vary quite a bit, but you shouldn't breed them too young.

MM

Answer 2

Prime breeding age for bettas is between 4-12 months. Females slow down egg production around 6 months and will produce smaller spawns. They can be bred after 12 months but they require longer conditioning and must be in optimum health before even trying. There is greater risk of the fish dying after spawning the older they are, and the less eggs will be produced the older they are, also. I know that in asia they start spawning their bettas at 3 months and by 5 months they don't use the females at all any more. And btw, they don't "lay" the eggs. The eggs are inside them. During the spawning process the male will wrap himself around the female, squeeze out the eggs, fertilize them and then put them into the nest he's built.