What is it that older disciples owe those who are younger and coming behind them? What is the best gift that mentors can give their mentees? What is the most helpful thing that parents and grandparents can do for their kids and grandkids? What should disciples primarily do for those whom they are discipling?

Diligently set about praying for them. This is what the Apostle Paul models for us when he prays for young Timothy. Here is one way of articulating his prayer from 1 Timothy 1: 15:
Gracious Father, for your glory, and for our flourishing, exhilarate, comfort, and quiet our hearts with the reliable, universal, historical, liberating, and personal gospel of our Lord Jesus Christ.
”The saying is trustworthy and deserving of full acceptance, that Christ Jesus came into the world to save sinners, of whom I am the foremost.“
1 Timothy 1:15 ESV
https://bible.com/bible/59/1ti.1.15.ESV
The Apostle Paul teaches us in this particular statement of the gospel that it is reliable (a trustworthy statement), universal (it deserves full acceptance), historical (Christ Jesus came into the world), liberating (he came to save sinners), and personal (I am the foremost, because once the Holy Spirit, convict us of sin, we give up all odious comparisons.
This insight is taken from John Stott’s devotional book, Through the Bible Through the Year.