I'll answer each part separately.
Firstly. I can't tell you how to sex it or age it on sight. I think that youngsters often have white on them, and they gradually lose this. Unless it is a ringed bird, checked by a ringer, you will not be able to tell origin or age accurately. However, often new arrivals are new bird.
Tawny Owl using box - sounds very hopeful. However this means that a Barn owl box nearby is probably not worth trying - Tawnies will attack Barn Owls close by. However a good distance from the Tawny, in an open area at the edge of the golf course may be suitable for either a Little Owl or Barn Owl box.
Little Owls like shorter grass for catching worms and insects, while Barn Owls need voles and other small mammals they get from grassy habitat that is long and wild. Barn Owls will rarely find a mole above ground, so they are not a good prey source.
Your extra items of 'prey' put out for the owls may begin to get eaten if out ready for the evening, but consider this carefully. Will an owl become dependent upon extra food? Will an owl start to associate your arrival in the evening with a food supply?
Best of luck, and congratulations on the owl. - BOCN.
I spotted a tawny owl in the garden last year and have made and erected two large nesting boxes fairly high up in a couple of large European Poplar trees. After just three days a tawny owl roosted in one box for three consecutive days. Fourth day and following three days it is roosting in the other box, How can I tell if it is a 2005 fledgling and what sex it is. What is the likelyhood of it nesting in a box..? Should I put up anymore owl boxes for Little owls and barn owls. Our plot around the barn is about two acres, but we have fields and trees and a golf course nearby. Moles are common in this area . I put out at dusk after the owl has left to hunt for the night, three day old chicks, on a high feeding table, but so far these seem to be food for the jays and magpies that visit the following morning. Should I continue this strategy of trying to feed the owl?