The best time to perform Aqiqah according to Sunnah guideline is on the 7th day after the birth of the baby or otherwise 14th or 21st day.
But if you miss the Sunnah date (7th, 14th, 21st day after the child birth), you may perform Aqiqah anytime after the Sunnah date. The best is to performed aqiqah prior to baleegh (puberty). However, if the child has become baleegh (reached puberty) or has become an adult, the parent is still liable to perform aqiqah for the child.