It would be interesting to set the age gap as a scale based on how much life you've already lived, (As it does for humans in RL, doubling every 7 years or so): At 18, dating someone who is just 2 years older is considered a major step, while at 30, being romantically interested in someone just 10 years older or younger is kind of "meh", and at 50, wooing someone 20 years older/younger is also kind of "meh". In RL human cultures, the age of maturity was usually around 12 years, and average age of sensecence was around 50. That gives us a 7^2 progression to old age, where the humans would be less interested in taking mates.
At 12, the age gap is very narrow: only other 12 y/o
At 19, the age gap is less narrow: +/- 2 years (17 through 21 y/o)
At 26, the age gap widens: +/- 4 years (22 through 30 y/o)
At 33, the age gap widens further: +/- 8 years (25 through 41 y/o)
At 40, the age gap widens further: +/- 16 years (24 through 56 y/o)
At 47, the age gap is very wide: +/- 32 years (15 through 79)
At 54, the age gap is too wide: +/- 64 years (Age 1 through 118) (It does predict the "child bride" culture though, in cultures where age of majority is not a barrier to marriage/mates.)
Using this, we can see dwarves live to about 200 before passing on, and are virile all the way through. The square root of 200 is 14.14, so lets round that down to 14 (196 when squared) and see where that gets us.
Age of majority: 12 y/o (no child brides)
At 12, only another dwarf of the same age will do.
Same until age 26 (12+14): Then its +/- 2 years: 24 through 28
Age 40: +/- 4 years: 36 through 44
Age 54: +/- 8 years: 46 through 62
Age 68: +/- 16 years: 52 through 84
Age 82: +/- 32 years: 50 through 114
Age 96: +/- 64 years: 32 through 160
Age 110: +/- 128 years: 12 through 200 (The full spectrum)
Now if both sets of dwarves are interested in each-other, then they get together and become married. As a dwarf ages, they gain status and titles and such, so its possible that a younger dwarf "marries up" for status or valuables, or maybe they just get hitched because they're there across a crowded ale-hall and that makes things just peachy. I'd love to see length of names be an indicator of status and therefore heritable.
Some synchronicity might happen at certain age groups:
Humans: Male vs female preferences in Europe and Asia have typically driven the marriage customs: females are often paired with husbands that are "too old" or "too young". Modern humans have a very wide age gap that is associated with dedicated companionship such that a 21 year old with an 80 year old "sugar daddy" is considered inappropriate, but a 25 year old female with a 40 year male is considered very regular. (even if the husband is closer to the parents' ages!)
I imagine the age of majority creates the beginning of the ability to produce offspring, and it is that period of being able to produce offspring in which lasting relationships are made. Currently, 60 year old women and 90 year old men can still procreate. So it is the range of procreation that may drive a culture's acceptance of committed relationships.