D&D 5E "Young" versions of Large beast companions for Rangers - how to do?

[Herobizkit]

How might one change the Large animals to "young" versions to make them qualify for companion status, and at what point might you allow the creature to "grow" to its full Large version?

 

[Andor]

Why not just give the young beastie the stats of a normal Ranger AC like the mastiff or panther, whatever seems closest, and the grow it (Panther is evolving into Smilodon!) when the Ranger reaches the appropriate level?

Unless I'm missing something.

 

[jadrax]

Why not just give the young beastie the stats of a normal Ranger AC like the mastiff or panther, whatever seems closest, and the grow it (Panther is evolving into Smilodon!) when the Ranger reaches the appropriate level?

Unless I'm missing something.

I don't believe Beast Masters get higher challenge pets as they level.

 

[Andor]

I don't believe Beast Masters get higher challenge pets as they level.

Ah, I don't have the PHB yet. Well then give a larger pet either when you think the Beastmaster (I'm not going to call him the BM Ranger) lags behind the Hunter, or when he pays for it.

You could let the PC give up a stat advance to change his beastie something more 'roidragy, effectively making it a feat.