I would wait until the bones are fully developed, around 2 yrs. We got our Rescue GS at about 15 months. When we got him, he had no muscle mass on his hind end. It took us about a year to get his weight and muscles developed to a healthy point. This was done thru walking/hiking, then eventually jogging. Last Fall I started him riding along side the bike. We worked our way up from a mile to 12 miles. Most of the time he is at a nice trotting pace. This was over the course of about three months. He loves it...when he knows he's going he starts prancing all around.
To help your pup become familiar with a bike, you can walk him several times along side the bike up and down your drive. I plan on doing this several times a week with our new puppy as he will need to learn this for the AD down the road. When we start to really train for it, it'll be natural (that's the thought anyway).
Good luck! Make it fun and your pup will love it!
Mr. Red - Rescue GSD (CGC)
Ms. Charlie - RIP 1-14-2014 Rescue GSD
Ms. Gypsy - Yellow Lab (CGC)