We do not yet have enough experimental data to answer the question precisely, but there have been about one per century in the Milky Way (at least, the part that we can see). We know that there are...
Depends on your age, the older the couple the more acceptable the age difference. For example, a 50 year old dating a 42 year old is fine but a 21 year old dating a 13 year old is just gross.