It depends on the gecko. It's worth a try for any gecko over 9 months and 50 grams in good health. You may not get a great result with an older gecko, but there's only one way to find out.
I think Matt wanted to know if he can breed older geckos, not how young you can start breeding them at.
I'm not exactly sure myself. I think it depends on the gecko. My geckos are all fairly young, so it's not something I've had to worry about in my collection yet. I think females are good until 9 or 10 years though, but hopefully someone else with a larger collection than me can give some better information.