My son became an Aeroplan member the month he was born!
All of his flights to date have been as a lap child, so no points from flights yet and therefore no status, etc. However, he has benefitted from things like the shop.ca first time customer special and other such one-per-person promos.
The nice thing about Aeroplan is that the points in accounts of minors don't expire until they turn 18 regardless of account activity.