Please tell us more to help narrow down what the problem is.
1) It is not receiving enough gas or the motor isn't warmed up enough.
2) You are not pushing in on the clutch when engaging the blades/lowering the deck or not pushing in on the clutch far enough.
3) You are engaging the blades/lowering the deck some place that provides too much resistance to the blades turning - such as grass that needs cut.
4) There is something binding in the blade/deck belt path.
5) You are attempting to engage the blades/ lowering the deck while it is in reverse.
I ruled out the clutch safety switch since you are getting it started. I ruled out the safety switch that won't let the tractor run if the blades are engaged on grade with to much of a slope (since you said older). It should be one of the above or some combination of them. With the information you have given I can't narrow it down more than that.