On my course at Imperial College I'm the second oldest at 39 in a class of 47 full-time people. The youngest are around 25, the oldest 52. I reckon the median would be around 30. Usually a few years' experience of work first is a good idea: Imperial claim they insist on 4 or 5 years.
10+ years of work may really help you relate to some of the issues that come up in the course, though - you'll remember stuff that has already happened to you, so it'll be easier to absorb apply the ideas than if you had learnt them first and seen them in real life later. But I'm sure at 30+ (unless you've spent years in academia) you'll get the benefit.