Ella Darcy from Department of Author Correspondence joined at ten in 1999; she's possibly the youngest agent ever. But she wasn't a field agent until the LotR fic explosion of 2001-2003, at which time she was anywhere from twelve to fourteen years old, barring the wobbliness of HQ time. She's from World One, too.

Fourteen seems to be the youngest age that a World One human is allowed to start going on missions. Obviously, non-human species have different ages.

There's the case of Mikaela Hamilton, who was hired as an agent at fourteen but persuaded not to go into the field until she was older... So there is precedent for unusually young agents joining the PPC and working in non-action jobs or as interns under experienced agents until they're old enough.

It's not necessarily a matter of popping her into the Nursery; her age is already pretty odd, and if she was preparing breakfast (with a stove no less) while being described as a "toddler"... well! Age might be more-or-less irrelevant for her.

She'll still be small until she gets older, though; and you probably do need to be strong enough to assassinate a Sue or restrain a possessed canon.

It would be interesting, actually, to have a child Agent from a world where children aren't particularly coddled, as they are in World One. I know Ender's Game is quarantined... We already have one from Hunger Games. Maybe the Giver trilogy? Not from one of the Communities, obviously; but from the more "primitive", wilder areas where children often have to grow up quickly; and you'd get the bonus of a possible minor power to play with, too.

The usual recruitment age is fifteen, with exceptions down to twelve AFAIK. Considering that the PPC is ALWAYS understaffed, Flowers will put her on the field at exactly the minimum age.