If the age gap between Chakotay and Seven posed a problem at that time (while both were human), the situation will be the same and even more complex between Seven and the Doctor, knowing that the latter is a hologram. I mean, even if the Doctor decided to alter his age to better seduce Seven, the physical appearance of Robert Picardo will not deceive anyone. He is getting old and it shows like the nose in the middle of the face!
In 90's
lately
Plus, again, only the Doctor really showed his interest in Seven for a potential love relationship, while Seven really saw him as a very good friend.I don't see why it will change now.