A.T. Robertson Commentary John 21:18

"Verily, verily, I say unto thee, When thou wast young, thou girdedst thyself, and walkedst whither thou wouldest: but when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and carry thee whither thou wouldest not." — John 21:18 (ASV)

Thou girdest thyself (εζωννυες σεαυτον). Imperfect active of customary action of ζωννυω, old verb, in N.T. only here and Ac 12:8. So as to περιεπατεις (walkedst) and ηθελες (wouldest), two other imperfects of customary action.

When thou shalt be old (οταν γηρασηις). Indefinite temporal clause with οταν and the first aorist active subjunctive of γηρασκω, old verb to grow old, in N.T. only here and Heb 8:13, "whenever thou growest old."