A good question!
I do support lowering the voting age to at least 16, and allowing more people to take part in the democratic process. I'm not sure that a lot of the arguments made against 16-17yos voting can't also be made about people older than that, and I think it would be beneficial if more people had the opportunity to cast their first vote while still living at home and able to be encouraged to do so (and educated about the process) by their school or college.
There are also ways that people below voting age can and do get involved in politics – they can join political parties very often at a younger age than the voting age, and there are institutions such as the Youth Parliament (and equivalent youth parliaments in the devolved nations, youth councils in local government etc) which are specifically designed to ensure young people can take part in politics and put forward their views in a formal, if parallel, setting.