No good way to really differentiate between a 2.5 or a 3.5 yr old. If you enjoy them, no need to sweat their age as they can still earn their keep as the elders of your expanded flock.The reason I asked is two fold.
First, I have three hens, I think they are one ISA and two Hyline browns. Were free from a friend who can't remember which batch they came from, so they could be mix of 2.5 or 3.5 year old.
Second, there is a lady selling very cheap hens that are 2-8 years old, and she doesn't know which is which!
2 to 8 is a huge range, I'd skip on those. You could probably guess which one is a 2 yr old versus an 8 yr old, but 2 vs 4 or 6 vs 8? Really won't be different.