It's worth bearing in mind that just because this virus has been around for 3 months, this doesn't mean it's been noticeable for 3 months, or important.
Let's say cases doubled every week:
Week: cases
1: 1 September
2: 2
3: 4
4: 8
5: 16 October
6: 32
7: 64
8:128
9: 256 November
10: 512
11: 1024
12: 2048
13: 4056 December
14: 8000
15: 16 000
16: 32 000
17: 64 000 January
I've knocked this up really randomly, but just to show that with exponential spread, something can have been around a long time, but only really come into its own in the last few weeks. Given that those numbers (rough and ready as they are, but the proportions will be kind of right) would apply to all ages, you can see that some thing could have been around since September, and it would only be in the tail end of December that we would get anything like meaningful data about a specific age category.
I'd say we're still in the "not enough data" phase of this new variant.