You can drive and get married at 17. You can work and pay taxes when you’re 16, have sex, leave full time education and are charged as an adult for trains and busses.
I’ve no strong opinions on the voting age but I wouldn’t say putting an X in a box requires any more maturity than driving a 1500kg lump of metal at 70mph. We already give 16 and 17 year olds the chance to make plenty of big, life changing decisions.