Drag each capital letter onto its small partner. Knowing that A and a are the same letter is one of the first things reading asks of a child, and it is pure recognition — no sounding out, no spelling.
A book uses both forms on the same page, often in the same word. A child who reads A but not a is stopped by half the alphabet, so pairing the two is groundwork for every page that follows.
About three to six. A child needs to drag deliberately and to recognise a few letters already; the game teaches the pairing, not the alphabet itself.