your right sim, under the UK immigration system when you turn 18 your an adult, but under the Euro immigration system your still a dependent til your 21.
i'm sure its when you submit the app and not when they grant the visa.
i think you should do it all ASAP, i think in theory if you get a visa for you and your son, and he is classed as your dependent what ever happens to you visa wise, also happens to your son even thou he's over 18 if hes still dependent on you (doesn't live an independent lifestyle, lives with you, doesn't work, is not married)
as he would be over 18 thou, you would have to pay 2 lots of fees for each visa,
i wish you well you might need it