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.
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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:
(ANSA) - Rome, July 18 - Italy is set to lower the minimum age for people to become Senators and to vote for representatives in the Upper House.
On Wednesday the Senate approved an article contained in constitutional reforms that will allow for Senators to be as young as 35, down from the current minimum age of 40.
The voting age will be 18 for both the House and the Senate - at the moment a voters had to be 25 before casting a ballot in Senate elections.
Now, as to the "why", I was not able to find any officially stated reasons, but one can speculate on the obvious:
All voting age restrictions are based on perceived ability to make "wise" voting decisions.
Since typically, upper parlament chambers are elected for longer periods of time, AND are meant to "temper" popular vote based lower chambers (e.g. in USA, the Founders meant for House to be representing the People, and the Senate, the States); it may have been felt that Italian upper house should have "wiser" people electing them. Italy does have an electoral tradition of restricting franchise, for example:
The Sardinian Statuto Albertino of 1848, extended to the whole Kingdom of Italy in 1861, provided for basic freedoms, but the electoral laws excluded the non-propertied and uneducated classes from voting. (Wiki)
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).