Apart from the fact that it's creepy as fuck, (and no, not all men in their 40's and older fantasise about shagging a teenager!) I wonder what the hell they can have in common? What do they talk about? Her collection of sharpies and gel pens? Her collection of Justin Bieber cuttings she got from Bliss and J-17 magazine? What does he buy her for a gift? An alice band with her name on it? Some 'Frozen' socks? Some earrings from Claire's? 
It's like on Doctor Foster; her husband who is 43, goes off with her friend's daughter who is 19. WTF? What can they possibly have in common? Aside from the obvious; she is probably firmer and more nubile than the middle aged wife, what else is there? What do they even talk about?
And what about in 20 years time, when she is 39 (still young and attractive probably,) and he is a pensioner? Is she going to want to look after him when his 55+ ailments kick in, and he isn't the sporty, fit, sexy older man she went for, but a grumpy, miserable, and boring curmudgeon, who just wants to sit in the house watching old 1980's comedy programmes that were on before she existed?
I know several women aged 40-42 who got with - and married a man who was around 20 to 23 years older, when she was 22 and he was 42-45 (ish.) And although they were a sexy and solvent and exciting middle aged man then, they are dowdy, miserable codgers now who think her job is to be the housewife and mother , and do sod-all in the house to help, and do nothing but grumble about everything. One woman is 39, and her husband is 61, and he is ill all the time, and she spends her life caring for him. Even their 11 year old son barely gets a look in, as his elderly and ailing father gets all the attention.
I would actually pity the girl @SLA1978, and I honestly think your ex looks like a desperate and dirty old man. And everyone else will think it too.