I was thinking about this more in terms of which makes you more able to cope with the stresses of doing a PhD. There seem to be a lot of posts about people feeling inadequate and not being able to talk to their supervisor/other academics.It got me wondering whether these people went straight from being undergraduates, and whether this affects your confidence, or whether it all depends on the individual and spending time doing something else before starting a phd makes no difference.
Personally I graduated 4 years ago and only just started PhD. I think the time off has made me more grown up and perhaps more confident in my knowledge and ability, however I've also not been a student for two years, and find myself procrastinating a lot.

Quick point ... It's important to remember that the PhD is a 'process', so everyone (regardless of discipline) experiences pretty much the same things (i.e. ups and downs) although these may occur at slightly varying times.

hi,
well i've done plenty of stuff outside of academia and am not very young anymore, and feel i have certainly gained something from those experiences which in some cases helps with the PhD. in other cases, it is tough because i am often "put back in place" and very much made to feel like a "student who has no idea of the real world NOR academia" regardless of my real background. so overall i would say, yes there are experiences you can benefit from for your PhD, but some things also make it harder, and for some things it just doesn't matter - it's hard no matter if you are young or old, have lots of experience or not.

I am planning to go straight from UG to PhD training but the studentship is four years and the first year is an MSc plus project so I am hoping that this will give me a chance to develop my knowlege of the specific area. However, I did take two years out between school and my undergraduate degree to work and travel, which I feel helped me gain some perspective on my career aims.
I certainly wouldn't want to go from undergraduate to postgraduate study without having had that break!

As a "mature" student I think that having worked in my profession for years and having those experiences under my belt have helped--though at times I wish I could have done this sooner...I think it helps to have some real world experience that gave you confidence, and to learn how to negotiate the dynamics that you encounter anywhere...but there is no complete right or wrong about when to do it, if you have the opportunity, go for it, because really the chance to do a PhD is a privilege, and if you turn it down it may be hard to get the chance again ( money, time, family commitments, etc)