Whilst hatching under a broody is pretty easy on the keepers, it isn't very reliable. We've had chicks eaten by mum, heard of eggs abandoned just short of hatching and had a lovely hen take ill and die whilst sitting (heart problem I think). If we do use a broody again we will make sure she is in perfect health and has fertile eggs of good condition. We put 10 under an Orpington, which I think was too many. 6 under a Wyandotte got a perfect hatch twice. If you put too many eggs down she can't cover them all well enough. Some will be cooler and hatch slower and will be abandoned to look after the first to hatch. So too many eggs reduces the numbers successfully hatched I think. I heard of someone put 17 bought hatching eggs under a broody and she abandoned 15 -expensive loss. Of course all this uncertainty stimulated the development of incubators.
I think 2 years old is fine. Just keep the number of eggs down to 6 WBF and candle them all for hairline cracks first. Don't mix breeds, as the inherent difference in shell thickness and egg size can have an effect perhaps -no problem in an incubator though.