23 year old indeed might be too much of a difference IMO + there is a big gap of maturity between 23 and 28 years old, but 28 year old is totally fine in relation to his age (unless he behave and feels like a 50-60 year old). I was with around 10 years younger gf, she was very attractive and had many relationships before but couldn't connect with anyone as good as she did with me.