I'm quite in favour of the voting age remaining at 18, and wouldn't be averse to returning it to 21. The comparison with female suffrage is spurious: disenfranchising someone until they reach a certain maturity bears no relation to disenfranchising someone out of naked prejudice. If we follow this batty logic we'd have to remove age-qualifications entirely.
The majority (and I emphasise the majority) of 16 year olds simply lack the life experience and intellectual development to vote. I look back at my own views at 16, even 18, in horror. While I reluctantly support a universal franchise (I can't think of any qualification that wouldn't be abused the moment it was introduced), within that framework I enthusiastically support keeping the quality of debate as high as is practicable.
The shocking decline in political discourse is readily evident to anyone comparing Victorian and Edwardian debate with today's, and I don't think we need to debase it any further out of some flawed notion of equality.