I suppose it would depend on the subject area.
I feel that those who study such subjects as art or theatre/performance, or even music, would benefit from not going into a master straight away, but instead have some time to independently develop themselves in the real world, outside of universities and academic structure. There is a developmental arch that people need to go through outside of school/uni in order to improve themselves further, and then perhaps take a masters later if they feel they need support to develop particular skills that they have touched upon.
But for other subjects such as business or law, I guess it would make sense to follow on with a masters either straight away or after a years break. It's a very competitive area I hear. Although there is this common notion that age equates to experience; qualifications and knowledge are still advantageous, and I wouldn't find it unusual for a person to be 21-22 and doing a masters in those kind of subjects, in fact it would make sense to me to do so. But I don't study those subjects, so my opinion may greatly differ from those that do.
I'm going to be 26 when I do my masters. I took two years out to reflect and do some independent work, and I found it very useful to do so. But other people I knew who did an undergraduate at the same time as me either went straight into a masters or straight into work.
Overall, I don't think there is an 'ideal' age, per se. It would depend on the competitiveness of the subject area/industry, and whether or not you feel you want to get a masters under your belt before embarking on anything else.
What subject area will you be studying, if I may ask?