So you have a German Shepherd Dog. There is no reason to think that the GSD is innately more aggressive than other breeds.
The bigger the breed the slower it is to grow up and mature physically and mentally;under six months for the tiniest, well over two years for the tallest.(Most Great Danes are suspected of not maturing mentally ever, but don't tell

)
What you describe sounds perfectly normal. Not just 'normal for GSDs' but normal for all breeds. (Chihuahuas do it; they do it to each other and bigger small dogs; but they find it devilishly difficult to reach your arm, so you'd never know !

)
This 'mouthing' or play- biting, part of the growing up process, and intended as play in the litter, can be very irritating ( and if my Standard poodle forgets once more and does it again she's dead meat; and at 9 months too

!). It can be alarming, particularly to non-dog people or children. The dog quite often leaves red marks on exposed skin; it has no intent to bite and doesn't close its jaws to do so, it's a play, just as with its sibling pups it was and would be.
Any basic book on dog training will tell you how to deal with this ( I must go out and buy the standard poodle one

).
Basics: First don't respond by shouting, smacking or pushing away. Step back, turn away and ignore. Second ignore a jumping puppy; praise a pup/ youngster only when all four paws are on the ground. Third, in the park, use an extending lead to control jumping up at passing dog-lovers. Make sure that everybody in the family does the same.
Use a leash indoors; when you hear a visitor coming clip on the leash. When the visitor comes in snap the leash if the jumping starts, saying 'no' firmly. The same practice is effective outside, naturally.
Don't let the dog jump up and greet you when it wants to. It should be ignored for 15 minutes when it jumps on your coming in until it calms down. Once it is sat then praise it calmly.This may puzzle the dog at first but at least it is consistent and not confusing to it.You'll notice that show dogs,not just obedience dogs, are trained not to jump up for praise until the owner encourages them to do it on a signal.( Then they nearly get knocked over by anything bigger than a setter

)
Now I've not mentioned the biting; stop the jumping up because that lesson lasts a lifetime. The dog will grow out of the play biting that goes with it. All dogs should know the command 'down' or whatever word you choose, to lie down anyway; but responding to 'no' in a tone of 'don't even think about it' when it comes to potential jumpers is a start !
[This message was edited by FredPuli on 02-03-04 at 05:11 PM.]