In some countries you can vote for one arm of the parliament at one age (say 18 years) and for the other at a different age (say 25).
Is there a rationale or is it merely tradition?
For example this happens in Italy.
As far as Italy, it seems to be merely tradition (but short lived one, as the current voting laws were established in 1946). I base this answer on the fact that they are, in fact, changing the 25 YO limit:
Now, as to the "why", I was not able to find any officially stated reasons, but one can speculate on the obvious:
Following e.g. the British Parliamentary system, there is an "upper" and "lower" house (House of Lords and House of Commons in Britain, Senate and House of Representatives in the U.S.)
The lower house is supposed to represent the "will of the people," while the upper house is supposed to represent the "establishment." The idea of having different voting ages for two is that you are a member of the "people" at one age, and a member of the "Establishment" at an older age. Despite differences in maturity BETWEEN people, everyone is (supposedly) more mature relative to THEMSELVES when older than when younger.
It is noteworthy that in the United States that there are different minimum ages for SERVING in the Congress (25) and in the Senate (30). This five year difference carried more weight at the time that the Constitution was adopted, when the average life expectancy was about 45, than it does today (life expectancy more like 75).