Depends. Some things are 6 and up because most don't have the fine motor skills to handle until 6 to do. Some things are 6 and up because it's for a safety reason. And some are labeled an age because of maturity. So if you think the child getting the toy can handle it for safety, maturity and fine motor skills at that age, then it would be okay.
I can tell you my mother in law sent my son all 6 and up toys for his 5th birthday. Most went in the closet since he couldn't do them due to the fine motor skills. The rest he isn't interested in because they are just over his head. Personally, I like to stick with the child's age unless I know for sure that they can handle something above their age range.