Sorry for my delay! (Thank KK for helping!) The seeds in the younger hay are tougher and the stems and leaf are greener (not as much now though) It’s less about first or second cutting, though technically it would be the first cut – it’s just not in the same way that Timothy is harvested. Oat typically can grow and mature and gets that golden color even before it’s cut, but if you want to cut it before it matures, then that cutting would be considered Young.
Typically it’s greener than Mature Oat, but while it is stored through the winter months, even the green oat will turn more golden but the seeds are still harder. It also has a little different taste that some bunnies like.
Young Oat though is not recommended for bunnies under a year as they don’t have the mature digestive system needed for Young Oat.
As far as allergies go, both hays would be equally okay as the stems and leafy parts are still the same texture – just the seeds are harder.