From what I’ve seen, average age for full-time is around 26 - meaning average work experience is 4 years. But this differs by program. Check Businessweek and U.S. News - both have online and print resources which will cover this, and most schools have this on their sites as well. Here is Northwestern full time info - age 28 with 5.3 yrs. experience: (and average GMAT of 704!)
http://www.kellogg.northwestern.edu/news/generalinfo/facts.htm
I already touched on some of the pros and cons of later vs. earlier. Of course there are cons of going too late as well - after young 30s with 10yrs pro experience or more, it may not be as useful to you personally, you’ll build less of a network (assuming you are running home to spouse and kids while others head for pub or all night study session), and 30+ will be an issue at recruiting time - and of course the recruiting opportunities are one of the main reasons you might consider a top tier program.
As for work vs. degree - obviously, if you are younger, they are going to focus more on the degree and judge you by your academic performance, energy level/motivation, and perceived potential based on tough interviews. And there are plenty of 23, 24 year olds in various programs, so some companies are set up to accept them and “build from scratch.” The more experience you have, the more they will factor that in - but mostly if the experience is with a known company and it relates well to your chosen major and the company you are interviewing for. Ex. If you are sketching shoes for 6 years for a no-name brand, then do a marketing MBA, then apply at a major B2B tech services company in Silicon Valley, your previous work experience is going to amount to less. So, think about how it will all connect and appear to others sooner rather than later - how will you “spin” your current work experience to get proper “credit” for it later?
Good luck and get working on that 704!