For me 10 years at max when speaking about relationship.
In that part of I'm sort of rational. For me relationship sort of means prospective changes of being together until the death us apart... and I don't want that day coming too soon... and I don't want to become damn home-nurse at the age of 30 or 40 and that might be the reality if I would marry man in his 40s or 50s when I myself would be in my 20s. Not necessarily but the changes would be quite high and men are quite good at breaking themselves. >.<
If there would be 20 year age cap he would already be 60, when I would be 40 and 70 when I would be 50... and the things would keep even freakier if the age cap would be 30 years.