If you are fit & healthy and able to follow the UIAA guidelines you are unlikely to have any serious problems (which advise that above 3000m you raise your sleeping height by no more than about 300-400m per day and have days rest every 3rd day). In 2011 at the age of 50 I followed these guidelines fairly closely - being un-guided I could follow my own program of acclimatisation - and I experienced nothing worse than shortness of breath - and a bit of 'periodic breathing' at night, which responded to half a Diamox... speaking of which you don't need to take Diamox to acclimatise but it can make you more comfortable. Don't even think of taking it instead of acclimatising.
I did notice that a lot of guided groups seemed to pay no or little heed to UIAA guidelines - and many of their clients either suffered unnecessarily or didn't summit - or both. For example one party of 6 led by a guide plus assistant entered the national park on same day as me - and went for the summit 4 days before the first day I felt I would be ready to try for the summit (when I did summit). Not one of them summited - and that included a really fit young woman who went like a train at all the lower elevations - but she hit the wall at 6000m.
So if you are looking for a guide I'd go for the one with the most flexible program - that gives you more options in event of bad weather also - at least one third of the days the viento blanco is blowing - and you don't want to be making a summit attempt in that! (some did when I was on the mountain - with tragic consequences)
I agree with Norris - Vallecitos is a great place to acclimatise - though I didn't go there before Aconcagua - but went there this year and had a ball.
If you are interested to read more about high altitude and expedition medicine here is the link to an article I wrote:
http://www.summitpost.org/expedition-medicine/675753Best of luck!
Mark