Is this a problem??

3girls

 18 weeks is no longer a puppy but an adolescent??? But that's still only 4 1/2 months old...

That's right.  We like to think of dogs as puppies until they are a year old, but in terms of their developmental stages, it's not quite accurate.  4-5 months is about the age when they lose their deciduous teeth and when adult dogs will start to discipline behaviors that were previously let go, because the little tykes have become doggy teenagers.  We often joke that pups just "lose their puppy license" - and sometimes, it's also when some adult dogs will be really brutal to puppies, so we protect them from dog park activity that gets too rough.  And, it's why most dog training schools separate by age.  Puppy class: 8-17 weeks, Adolescent: 18 weeks to social maturity.  The reason the cutoff is made there is that pups below 17 weeks are generally within the optimal socialization period, and we use different methods to train them.  Adolescents may need additional assistance if they have not had any activity off leash with other dogs before that time.  Also, while you can "rehabilitate" a dog somewhat, there is not an unlimited potential for that, and dogs without any form of social feedback are more often handicapped for life compared to normally socialized dogs.  We often find that dogs that are still nippy at five months of age stay that way, at least to the extent that they take treats with a hard mouth, and are candidates for flat hand feeding.  That isn't to say that you cannot teach an adult dog to "leave it" or install an "easy" cue, but the default behavior they offer, without human intervention, is often to grab at things.