Who satisfieth thy mouth with good things; so that thy youth is renewed like the eagle's. Psalm 103:5
There is, literally, a fountain of youth available to the believer by faith. We bless the Lord, as this Psalm begins with, and we're immediately recipients of this benefit of 1Peter 3:10, if we're aware of it and expecting for it all of the time as we bless the Lord.
His multiplying anointing doesn't stop at the bread Elisha multiplied in 2Kings 4:42-44. That anointing will multiply your longevity and your strength, give you youthfulness, and rebuke the devourer from your body. Psalm 103:6 could be read with the fifth verse to strongly imply that aging is Satanic oppression.
Verse 7 says He made His ways known to Moses. Moses is the one that lived to 120 in better strength and better health than most teenagers among us. Joshua only lived to 110 years old in Joshua 24:29 through meditating on all that Moses ever said (Joshua 1:8), but Joshua lived a much, much harder life than Moses did and was in very violent physical combat in his 80s, and perhaps beyond with whatever came up.
At the age Sarah gave birth, generations earlier, Joshua is running through the woods as very nearly a pre-historic Rambo! That man kept his mouth full of an inferior Covenant established upon inferior Promises, circumcision, and bloodletting from animals. Yet, Lord Jesus is our Sabbath and the true Bread from heaven for the renewal of our youth like the eagle's.