The feat (as written) can be taken at L3 but, because of level limits, you can't recruit a squire until L4. This house-ruled version permits you to recruit a L1 Warrior when you first take the feat, who acts as a squire and student.

Trainee Squire

A Trainee Squire is a a young person who has received basic military training somewhere, who has agreed to serve you in return for suitable training and experience - a sort of military apprentice. They look after your weapons and armour, run errands around town and do the mundane work around you camp - it return for some training, time to study and experience of the wider world.

If you take a normal warrior as your Trainee Squire, they they will transition into a L1 fighter when your PC reaches L4.

If you want a trainee squire who will transition into a different class - see the Warrior Archetypes page.

When your character progresses to L4 you may transition the squire to: Barbarian, Fighter, Paladin, Ranger, Cavalier or Magus, as appropriate for their archetype.