For everyone dismissing the skin aging issue, don't. You might look fine, but one action of minoxidil is to increase collagenase, the enzyme which breaks down collagen. You might still be producing collagen at a higher rate than you're breaking it down, but not everyone is. And as you get older, you produces less and less. So minoxidil does cause skin aging, but it will accelerate skin aging compared to the rate your skin would have aged without it.
That said, retin-a inhibits the production of collagenase, so it seems like it would be a good thing to add. Whether it can completely cancel out the affects of minoxidil specifically is not known, but it's sure going to be better than not using it.