Talking to friends in Canada, their nine year old plays hockey on the local lake in the winter. Matches are only cancelled if the temperature drops below -25C.
Personally, I won’t ride road any more than necessary below 0 as I find it hard to keep circulation to fingers and toes (and often another part of me that suffers from wind chill) but off road I’ve been comfortable down to -6.
We had -25 on a trip to Lapland and after 3 hours at the activity centre I found myself struggling to breath at all deeply. I’d been very warm playing on the ice scooters. I’m not sure I’d want to ride a bike very far in the fur lined romper suits they gave us to wear.