Actually as long as your child is under 24 months old AGE has nothing to do with it. It is about weight, as the bassinets have a weight limit, and each carrier is a bit different, IIRC they range from 10-16kg depending on carrier. IIRC TK is 10 or 12kg depending on a/c, although I do recall that the bassinets themselves are rated up to 20kg.

On another subject, I wince when people refer to seat assignments at booking as a "perk". This really should be standard, especially for elites............Oh how the airlines have beat us down!