When she's 21, 26 might not be too old. But at 16 a 5 year difference is WAY too much.
However, this is hard to handle. You don't want to drive her to this guy by telling her she absolutely cannot see him.
Explain to your daughter and your husband, it's just too big of a difference. He can vote, drink, and have sex legally. It's wonderful he sees your daughter as mature and interesting. However, at this time he is too old for her.
Also, think about the guy... if he touches your daughter, even if she wants him too, it is illegal and he can be convicted of a crime and marked a sexual predator for the rest of his life. He's 21, she's under-age, and charges can be pressed against him even if none of you want to file charges.
A 21 year old is not likley to be happy holding hands for very long.
Thanks for asking this question. I am now considering if I will let my almost 16 year old work.... yikes!