I'm more bothered by the geriatric politicians in various democracies than I am that you're missing out on some amazing politician in their twenties.

The UK has a practical minimum of 18 for Prime Minister (technically there is no minimum but practically there is) but realistically never elects a PM under 40.

For British Sovereign there is also no limit, any particularly young Sovereign has effectively delegated to a council of regents historically. In practice this is also unlikely - although in theory of course we are two untimely deaths from a 12 year old taking the throne.