You can age a sheep by its teeth. Lambs are born with eight milk teeth; at about 12-15 months, the first two permanent incisors erupt in the front of the mouth; the next two at about 22 months, one on either side of the first two; the next two at about 2 years 3 months and the last two about 2years 9 months to 3 years. As they age further, eventually they will start to lose teeth through wear and tear.
You can eat them at any age, I suppose - over two years old is classed as mutton, I think.