The number of years doesn't matter as much as the age of the youngest partner.

If one partner is 21 or older then I think the younger person in the pair should be either 18 OR out of high school (I graduated at 17, for example). Because anyone considered a "minor" in a relationship with an "ADULT" could spell trouble.

IMO, once you are out of high school you should be responsible for making your own decisions, and if that includes dating an older person, so be it. I met my future husband when I was just 17 and he is 18 years older than me. My parents were leary, but I was heading toward college and was a responsible person.

So, in this case, 16 is a little young for a 21 year old to be in a SERIOUS relatinship, but that is not written in stone,

If they are willing to proceed slow and prove they are a responsible couple I say try not to be uncomfortable with the age difference. In a few more years, if it lasts, the age difference won't even be noticable.