I am now 50, had a failed microfracture on MFC in 2007 and MACI in October 2010 so 16 months ago. I have not had the expected results and although walking now, and only since 10 days ago doing staircase "normally" I still vacillate between thinking that possibly I'm just slow in recovering(despite constant setbacks still improvement such as the steps) or that it has failed. Certainly I feel that my muscle strength and altered gait for so long prior to the procedure has had a negative impact( despite the daily rehab) I still battle with muscle imbalance which affects normal movement and glide over the condyles and hence impacting negatively on the graft site. I cannot walk for longer than 30 minutes or stand still for longer than 10 minutes before I need to sit again. Therefore cannot still do normal things like shopping, cooking a meal without planning and pacing.
From other blogs it does seem that younger people with sports injuries seem to do better . I think its related to their muscle strength and fitness as well as a more active approach sooner, as apposed to conservative management ,by Orthopaedic surgeons in younger patients.
I think your baseline muscle strength is important and if your other leg is poor (which needs to support you in this long rehab) it may prove difficult.
I certainly would get another opinion from someone experienced in ACI. From my physio who recently attended a talk by an OS they are starting to do ACI as a single procedure arthroscopically using your stem cells harvested from your bone marrow mixing it with thrombin to make it sticky and then gluieing it into the defect. This sounds promising to me and she will be sending me to this OS for an opinion as I would rather try this as an alternative before considering a major knee op again. Don't know much more about it at the moment, but it's certainly a good starting point for you to do your research on. Apparently it's only become available over the last 6 months.
Hope this helps