If you mean yearlings, 2 year olds, 3 year olds etc. These generally race from mid April for 12 weeks, prepared properly they'll not body moult in this period. Depending on what system raced you try to get them to throw their first flight as late as possible. They'll go into the body moult on or around the casting of their 5th flight which is to be avoided whilst racing. Its regarded they'll cast their first flight sitting their 2nd set of eggs (around 10 days of sitting). Natural fliers will pair up as late as March to hold back the moulting process. Widowhood fliers pair up much earlier but don't allow them to sit their 2nd round of eggs after 10 days, again to stop them casting their first flight. Darkness with youngsters ensures you race them in the best possible feathering, forethought & preparation with old birds ensures the same.