Limiting a pup to shorter hikes is definitely a good idea.
Although smaller dogs tend to mature more quickly, most sizes of dogs are not yet physically mature until at least 12 months of age. Large and giant breeds may be physically immature until 2 to 3 years of age. Generally, the larger the dog, the longer it takes them to mature. Labs are typically considered a large breed.
Until they are physically mature, there can be a lot of stuff going on orthopedically, even for an older pup. The growth plates probably aren't closed yet, the skeleton isn't quite done developing, and muscles haven't matured. Like human teenagers, they may be clumsy because proprioception can't quite keep up with their changing bodies.
These are the guidelines I try to follow when bringing along a young hiker dog. My experience and research lies with the giants, specifically livestock guardian dogs, which are among the smaller of the giant breeds.
0-6 mos.: no formal exercise. Lots of play, foundation for training. Avoid slippery floors (strains young joints) and jumping off things (too high impact).
6 mos. - 12 mos.: Light exercise, in the 1-5 mile/day range broken into twice daily walks, terrain ranging from easy to medium. The longer the mileage, the easier the terrain. Although some running may occur, regular trail running is to be avoided because it is too high impact.
1-2 yrs: light exercise, up to 10 miles broken into twice daily outings, again, higher mileage requiring easier overall terrain. Running with the dog at a working trot can occur, but no more than 1-2 miles at a time.
2-3 yrs: up to 15 miles over any terrain. May occur in one outing, reduce mileage for more difficult terrain. Running can begin regularly, no more than 2 miles/day.
3-4 yrs: up to 20 miles over any terrain. Running can be increased to no more than 5 miles every other day.
>5: Let the dog set limits - some dogs will happily do +30's, others are naturally couch potatoes.
Although I try to keep terrain easier overall for a younger dog, this is definitely the time to start introducing them to difficult terrain in short stretches. Just like socialization and training, they need to learn what steep slopes are and how to do rocky scrambles. The trick is to keep these difficulties very short and very positive for the dog.
I know, this sounds like a limiting schedule but... for big dogs, soundness is everything. If you stress them too much at too young an age, they may end up paying for it with decreased mobility as they age. Thankfully none of mine have been diagnosed with dysplacia, arthritis, or other orthopedic diseases. It's impossible to say whether this is due to my caution and/or other factors.
Only one of my dogs has been diagnosed with an injury. Dugan four-legs was recently diagnosed with a partial tear to the ACL in his right stifle. As far as I'm aware, this injury did not occur on a hike or run.
Apologies - stepping off soap box now.
