I think that the point of being 18 is to be young and stupid. You are supposed to make mistakes. What would be the point of going back to make all those mistakes again or to go back with the wisdom of being older and not live life foolishly. Making mistakes is not fun, but it has a value and to live life without the recklessness of being a teenager defeats the purpose of going back to that age again.
Not everything has gone right in my life. Not everything is the way I would want it in an ideal world.
BUT
I have a husband and a beautiful little boy. If I had done things 'right' when I was 18 who is to say that I would have them now and that life would be 'better'.
You have to live life as it is and enjoy the moment whilst it lasts and look back fondly but not longingly. If you waste your life thinking about the what if and if onlys you let it consume you and it stops you make the most of the here and now.
So I wouldn't go back to being 18 as that would mean I took for granted the good things in my life and was too happy to gamble that all away.
Life isn't perfect, but I wouldn't want it to be. Its the journey not the destination that makes it exciting.