My first husband was 27yrs older than me and didn't last long (various problems including our ages), the second, 10yrs older, still didn't last but not down to the age gap this time. After those I had a partner who was only months older, we lasted about 12yrs but eventually bad things happened and I left him.
My current partner is 4yrs older, doesn't live with me and we seem to be muddling along pretty OK, so I think I've found my 'correct age gap' and maybe geography gap too!
My youngest child came home 6yrs ago (age 18) and said he was in a relationship with someone only 17months younger than ME, giving them a 26yr gap, I didn't dissuade him (how could I, it's his life and he has to live it and learn from it) but I used my experience and explained a few of the issues they might face and said that I would be here for him if he needed help or advice. Over 6yrs on, they have their ups and downs, I think we're all surprised it's lasted this long but like everyone else, they muddle through and get on with life.
My conclusion is that people manage age gaps differently. It takes a whole variety of people and ages to get on in this world and no-one can say that one age gap is right and another wrong, its different for us all.