Hi this is the first time I've seen your post about betta fry and I wish I could have helped when you were asking about feeding them.
1st fry food should have been infusoria which you should have been cultivating at least a week before attempting to breed the fish.
2nd fry food would have been microworm, which you can buy in or start off with a culture, once again this should have been bought in a week before attempting to breed.
3rd grindal and bbs but I don't even attempt to feed them this for at least till the second or third week of their life when they are big enough to take it.
As for the age of their parents, after around 12 months you have the possibility of deformed fry or non fertilised eggs, the older the parent then the more chance of a small hatch rate or deformed babies, something any good breeder will try to avoid.
I hope this helps and if anyone has any more informative advise then please correct me, but any breeding of fish needs at least a good bit of planning before hand, especially if you plan on breeding fish that have so many fry.
Good luck with your next breeding attempt.
By the way water changes should NOT be attempted, just water top ups and then only very gradually over a matter of weeks.