I think it is a case of telling a little white lie, as they dont ask for proof of age or anything.
I was honest, and said my daughter was nearly 9 and could she go to Wavelength? The answer was no sorry!!
Friends of ours came out 2 weeks after us and son was also 8, she just registered him for Wavelength, cos he wanted to go with his sister and didnt say anything, and they never questioned it at all.
I would say they are all 6, as the activities at that age should be virtually the same, and my 8 year old only went once to the 6 -8 groups because she said it was too young, so I would imagine your 5 year old may find that too if he is in a group full of 3 year olds!!