There obviously cannot be an age cutoff point. Each case, like each human before God, must be adjudicated on its own merits.
That 25 year old with a fatal disease? What of it? MS? MD? MLS? These are long term diseases. The Pope had long term illnesses and was a beacon to the world in how to live with progressing terminal illnesses. Maybe the illnesses are some of God’s testing for us.
I think that it’s more about how you journey through life. When God calls one home, I doubt that He gets too many busy signals, but the methodology of His calls may vary from person to person. Both of my parents died of cancer and in each case, they knew when they were going to die before it happened and they accepted it and prepared to meet God. My grandfather knew it about a week before it happened to him.
They didn’t bring it on; it was there and it was going to happen. A far cry from killing one’s self for whatever motivation.
If you are very, very ill, and elderly, the health care personnel will automatically shove a feeding tube down your throat and fill you with Ensure or some other godawful concoction. Because you will not “get better” without nutrition. If you refuse, they will say you are crazy and try to give you antipsychotics. Our hospitals today are fearful of lawsuits.
So you have to make a choice at that time.