The haircut "problem" is more that it never changed than anything else. He first appeared in New Teen Titans #8, which had a cover date of June of 1981, so his character was created sometime in 1980 and having that kind of hairstyle wasn't exactly out of place then. As the decade went on it started to get a bit odd. It's also been suggested his appearance was based on comic vet Len Wein.

As for the age difference complaints as I've said before other than Beast Boy the Titans didn't particularly come across as teens to me anyways, so it doesn't seem that much of a problem. Donna and Dick certainly were involved in professions in their civilian identities where most people would have problems taking a 19 year old seriously in.

Honestly a lot of the "oh that creepy Terry Long" talk comes across as being as much the result of people repeating the idea over and over again until everyone became convinced of it as what was actually done with the character.