I wouldn't want to go early, I don't think its a good move. It puts a lot of pressure on a young person who will not be able to fully fit in socially. It would be embarrassing to not be able to go out with everybody else because you can't get in bars and clubs.
I think if you are mature you are more likely to be able to hit the ground running when you get to uni. In first year people are at different stages of development, both on the practical side (cooking, washing, dealing with 'situations' like landlord issues without relying on parents to sort it out) and on the emotional side (being able to cope with homesickness, being robust enough to deal with relationship setbacks). A problem some uni students have is they can end up academically underachieving because other problems take over their lives.
Probably the biggest risk is relationship fallout. For some young people university is a time of having their first real proper relationship (or maybe they got in that relationship before uni and it ends at uni) and so its the first time they have to deal with serious heartbreak. Also some students get to university and struggle with rejection: they see their peers getting coupled up and they hoped they would have these experiences but nothing happens and they get no interest from people they like: this can hit their self esteem. At the opposite end of the spectrum, some students get to university and find they are popular and that becomes a distraction, as in the whole battle of self-discipline between studying and partying, its a lot harder to avoid partying if you have a credible expectation you are going to get lots of attention and end up with someone at the end of the night.
Being a bit older doesn't remove all the above problems but it means you are more likely to be able to deal with it.
If you go as a mature student you can still fit in with others socially but you have to remember that people will have different expectations of you, even the younger ones. There was a 25 year old at our uni who was notorious for a) not turning up to half his lectures and b) continuously going on about how he was 'shattered' by a relationship break up with an 18 year old he'd met there. Whilst this would have been accepted as normal from an 18 year old, people did wonder on the first one, why he had bothered giving up a job to go to uni if he didn't care about the work and also being the lovelorn romantic with a broken heart might have sounded cute for an 18 year old guy but people did think a 25 year old should have been able to handle it better.
As a mature student you have to try and avoid the impression that you are like the kid that stayed down a couple of years at school for being slow: if you are mid 20s and showing the life experience development of an 18 year old then it looks a bit weak. If you are mid 20s and able to take charge of and manage situations then you can get on well with the younger ones because they will look to you as a responsible figure that can sort things out and that works in the social dynamic.
The final thing to be wary of if you are a mature student is the relationship side. At unis there are loads of relationships of different ages and you will find things like a late 20s/early 30s PhD student seeing an undergraduate aged 21/22 etc. However if you are a mature student male and you are continually hitting on fresher females it is going to look odd and get people talking, if you are hot and generally successful with these younger girls it will be seen as sleazy, if you are not successful and just trying it on with younger girls and getting rejected then you will become a laughing stock, sorry.