While I can't add much to the age question as such, my poor Engineer's brain couldn't resist the challenge of figuring out how many years have passed in the first 21 volumes.
The answer is eight years! (Slight chance it is nine, but eight seems pretty solid.)
Using seasons, holidays, and weather, the break-down ('V' means 'Volume', 'M' means 'Mission') is as follows:
Year 1: V-01 - (8 months): June - Christmas.
Year 2: V-02, V-03/M-5
Year 3: V-03/M-6, V-04
Year 4: V-05, V-06, V-07 - (very busy Spring confuses things slightly here).
Year 5: V-08, V-09, V-10, V-11/M-3
Year 6: V-11/M-4, V-12 - Umi (18) and Gojyou (26)
Year 7: V-13, V-14, V-15, V-16
Year 8: V-17, V-18, V-19, V-20
Year 9: V-21 - (~3 or 4 months)
Note #1 - In year 6, it isn't clear whether Umi and Gojyou are 18 and 26, respectively, at the beginning of the year, which means they would have turned 19 and 27 by the end of the year; or, if their 18th and 26th birthdays occur during this year.
Note #2 - In Fukuoka, Mayors can be recalled by the people, and special elections subsequently held. This messes up the '4 years between elections' rule.
I have detailed notes if anyone is so foolish as to ask for them!