The answer to: “is it morally wrong for a 28 year old man to date a 16 year old” is “it depends”.
However; the answer to: “is it **intrinsically **wrong for a 28 year old man to date a 16 year old” is “no”.
I think there has been confusion, because people have answered one or both of these questions without making the distinction.
The second question, asking whether it is intrinsically wrong, simply elucidates the fact that there is no strict moral boundary for two persons of such disparate age to have love together.
However, the first question, asking whether it is morally wrong, is more instructive, because it brings in real life. The fact is that in the vast majority of cases, this relationship would be, at least, highly inadvisable, and at most, sinful.
(Everything I say following this point assumes that the couple actually is acting morally and not kissing/having relations without getting married.)
The reason for this being unwise is that the 16 year old is so young that she is not matured. I don’t say “matured” just meaning maturity in behaving like an adult. I mean that her body **literally **has not yet reached maturity, and her hormones are going to continue to be changing drastically for about 8 more years. This means that feelings she has now, or chemistry she may have with the 28 year old man, may very easily change over the next few years. It is unwise for her to jump forward like this making such big decisions when she does not have the mental or physical maturity to do so with good judgement.
Scandal due to cultural norms is also a concern. Also, the fact that the man is so much older means that their relationship may have the attitude of father/child rather than two spouses shoulder-to-shoulder.