To whom did Paul write, "Let no one despise your youth"?

Answer: Timothy

Paul wrote to Timothy, his "true child in the faith," urging him not to let anyone despise his youth but to set an example.

“To Timothy, my true child in the faith: Grace, mercy, and peace from God the Father and Christ Jesus our Lord. Let no one despise your youth, but set an example for the believers in speech, in conduct, in love, in faith, in purity.”— 1 Timothy 1:2, 4:12 (BSB)