User blog comment:Sharaku Jr./Adult GT Goku?/@comment-5035900-20181102012701

Even with that handicap, GT Goku can still go Super Saiyan 4, overcoming the wish-induced age and its physical limitations. Plus, considering how Super Saiyan Blue and Ultra Instinct are, I doubt the outcome's really going to be determined by GT Goku being a kid or an adult.

I'd say go for the kid form just to mix it up visually between the two Goku's.