Handle he's birds race marking, are yours heavier or lighter, are they of a small type/lighter framed. A couple of things that may help answer your question.
When you take them of you'll start seeing them moult their first flight. At around 10 weeks after the day you take them of they'll start body moulting again and they're past their best. Race programs differ so its a case of working back say 12 weeks from your last y/bird race to ensure they're in good nick throughout the program. The flip side to this is you may find one or two might hold their last flight into the following years old bird racing and cast their last flight when they cast their first putting them out of action for a few weeks.