According to canon sources (novels) Jaina was 3 when Stormwind was sacked in the First War.
If we take the intro of WC2 as canon vs. the Tides of Darkness novel, that puts Jaina at 9 when the Second War started, 11 when it ended and she went to study in Dalaran, 17 when she was apprenticed to Antonidas and 18 when she started her Romantic relationship with Arthas.
According to the Lich King novel, Jaina is 1 year younger than Arthas, and the 3rd war broke out when Arthas was 24, putting Jaina at 23 during the events of the 3rd war.
It is believed, though unconfirmed, that Vanilla WoW starts ~5 years after the Third War which puts Jaina at 28 at the start of WoW, and that the events of WoW itself take place over roughly a year per expansion, putting Jaina at 36.