Usenet wrote:

That's the question, what's your answer?

Supplementary to this:
Would you want the plate(s) lit at night? How does one do this (creative
use of EL-wire, perhaps)?

That would depend on your reasons for licensing the bike. If it were to
prove legality or insurance or permission to be within a certain area,
then a sticker that is large enough to be seen by whatever appropriate
authorities would be sufficient. Perhaps suitably coloured on the seat
post?

If you wanted it to to be a 'number plate' in the same was as for
motorised vehicles, then obviously it would need to be bigger.

Should all age-groups be licensed?

That's a different question; you asked a question based on a contentious
assertion "if bikes were licensed", but this doesn't answer why any
should be licensed.

Why are you licensing them?