Don't bother bringing loads of stuff with you, see what everyone else has once you arrive... remember that you are living with other people, they bring things too! Also, there are loads of things which you think you'll need that you'll never actually use, one of my flatmates had pretty much every utensil under the sun and probably used 3 of them during the year. Its better to live in your flat for awhile, then if you're lacking anything do a shop specifically for that. There is a shop on Nicholson Street called Edinburgh Bargain Stores and they sell practically everything you could require, and also a Woolworths on Lothian Road, there is no desperate need for Ikea.
Age wise, yeah they mix you up although some flats are 'older' than others. There are specific residences for mature students, so most people in the 'normal' residences are of 'usual' university age. Everyone in my flat turned 19 during the year, but most people had a mix of 17-21/22 year olds as the exchange students tend to be older. Age is nothing but a number, no one cares how old you are... unless your Scottish and not legal when you start, but there are ways around that so don't worry!