Just because the calendar says you age up doesn't mean you can't run W21! Case in point, Pavlina Brautigam.
If you register for a national event and it lists you in W35 you can switch it yourself in the drop-down.
That means you're the same age as our older son....
And I do understand that you feel sad about not running with W21 with the current crop of competitors, but if you're up for W21 you can run against the younger folk and feel good when you win. :-)
I squandered several years of prime physical condition not knowing that orienteering existed. Sigh.