I understand that it is common around the world for women to marry men who are older than them - though there are many individual exceptions (I know a couple where she was in her early forties and he in his early twenties when they married, and ten years later they're still together and as far as I know happy).

The link below indicates that in New Zealand the age gap between first-time brides and grooms is currently about 2 years, and was 3 years in the 1960s.

One theory behind why this age difference persists is that males take longer to mature than females. Which would explain the wider age group back in the 60s when the age of first marriage was for brides about 20 years old and for grooms about 23.

The other side is that the available pool of potential marriage partners was smaller in Regency England than it is nowadays. According to the Mysterious H.C.'s post on incomes and status, there were about 27,000 families in the nobility and gentry in Regency England. And travel was a lot more expensive. So the pool of men available to fall in love with was more limited, so people could be less sub-consciously picky about appropriate age differences.

Plus nowadays quite a few people meet through connections that are more split up by age, eg at university while there were quite a few mature students, most people there were within a few years of my age. Or people tend to go on their OE in their early twenties. In Regency times, women didn't go to university, so men and women would normally have met either by growing up together or at balls and parties attended by the whole range of ages. So there were relatively more opportunities to meet someone of a quite different age to you.