There's a reason for this, I'm a bit vague on the details, but when Parliament took control of the country from Charles II, one of the agreements was that the monarch would keep out of political affairs.
It was 1928.
1928 equalised it to everyone over 21
Like most things, Suffrage was a scale - before 1835 most men didn't have a vote either, working men got it in 1867, 1918 all men over 21 and property holding women over 30 got the vote, in 1928 all women over 21 and in 1969 everyone over 18. In devloved areas like Scotland suffrage is even lower (16).
Unlike in the US I don't think it's ever regressed
Only for certain elections -- local and Scottish parliamentary. You still have to be 18 to vote in elections for Westminster; the same was true of European elections when they were still a thing in the UK.