Depends on temperament and surroundings imo, I stated my pup at 4/5 months recently 4/5 miles on the flat, gently, couple times a week, but, I've got a fairly quiet run on my doorstep although pups quite outgoing lol try walking it while your pushing the bike first so it gets used to being alongside, Don't overpace or distance it, they should see the bike as a source of fun, I believe. HTH
Cheers, D.