i dont breed my females til they're about 5-6 months old & around 250+ g if possible too.
i also stop breeding when they reach 1 1/2 yrs.
from what i've read theres nothing to support the idea that old rats cant produce...they can just their success rates are lower.
Temperature is also a big factor too.
I've found that if its hot the rats wont breed & i run into this issue in the summertime months. Once i started keeping the room cooler, i started getting better success with them &
lastly what they're fed is a big key factor in their overall health.
i switched off of lab block food & now i feed a 50/50 mix of a store bought food as well as my own homemade mix that i put together for them that includes cereals, rolled oats, dried fruits & veggies & sometimes sunflower kernels. i dont feed the junk treats that the stores sell. thats all unhealthy crap. period.
My rats treats are either fresh or home dried fruits like apple slices. My seniors get 1 dried prune every week just to keep their digestive systems running smoothly.
Anyhow, After switching to this food mix i found the rats had better energy levels & their fur is shinier & softer & they're just healthier looking overall.
Healthier adults produce healthy babies that become more nutritious feeders .