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 as a squire when you first take the feat.
Trainee Squire
A Trainee Squire is a level-1 Warrior 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 your camp - in return for some training, time to study and experience of the wider world.
When you reach level-4 their training will be complete, and they will mature into a class that grants proficiency with all martial weapons as a class feature, as per the original feat. However, existing skills, feats, HP etc do not change, unless they need to be reduced to meet the requirements of the new class.