Large age gaps in relationships

So I've been raised with a father who's about 30 years older than my mother. It didn't work out between them and it's been drilled into me my whole life that age gaps ruin relationships. I know 30 years is a bit much but I also know there's plenty of people out there in happy relationships with a large age gap, so my question is; how many years between couples do you believe is acceptable and when is an age gap just too big for a relationship to work?