I'm going to get right to the point on this one. Satan will do anything to twist, skew, or in general just stop what God has planned. Probably the most prominent way that I have seen in my life is discouragement. He tells us what we can live up (or down) to be. That we don't make the cut, etc.
Of these things he prompts us most to believe that our age limits our abilities. To the young 'you are to young, you don't have the experience yet'. To the old 'you are old, and can't accomplish anything with the time you have left'. To the middle aged 'you have to many other responsibilites, family, work, friends'.
But if we look at history there is no age bracket for world changers! If you are old, at least you can pass your passion and experience to the younger. To the middle aged, God will take care of you as you take care of his work. To the young, how to you gain experience? Not be walking, but rather from falling.
Kind of a random jump, but DO what God has placed on your heart. There inlies fullfillment, joy, and passion.