Your analysis looks good.
I still think that superior aetas here means “previous life,” not “the last stage of life.” Ironically, Lewis and Short would allow either meaning:
B Trop.
1 > Of time or order of succession, former, past, previous, preceding> : superiores solis defectiones, Cic. Rep. 1, 16, 25: quid proxima, quid superiore nocte egeris, id. Cat. 1, 1, 1: refecto ponte, quem superioribus diebus hostes resciderant, Caes. B. G. 7, 58: superioribus aestivis, Hirt. B. G. 8, 46: superioribus temporibus, Cic. Fam. 5, 17, 1: tempus (opp. posterius), id. Dom. 37, 99: tempora (opp. inferiora), Suet. Claud. 41: annus, Cic. Verr. 2, 3, 18, § 47: anno superiore, id. Har. Resp. 8, 15: superioris anni acta, Suet. Caes. 23: in superiore vitā, Cic. Sen. 8, 26: milites superioribus proeliis exercitati,Caes. B. G. 2, 20: testimonium conveniens superiori facto, Hirt. B. G. 8, 53: superius facinus novo scelere vincere, Cic. Verr. 2, 5, 44, § 116: superioris more crudelitatis uti, Nep. Thras. 3, 1: superius genus, mentioned previously, Plin. 13, 25, 48, § 146: nuptiae, former marriage, Cic. Clu. 6, 15: vir, first husband, id. Caecin. 6, 17.—
b Esp., of age, time of life, etc., older, elder, senior, more advanced, former: > omnis juventus omnesque superioris aetatis, Caes. B. C. 2, 5: aetate superiores, Varr. R. R. 2, 10, 1: superior Africanus, the Elder, Cic. Verr. 2, 5, 10, § 25; id. Off. 1, 33, 121: Dionysius, id. ib. 2, 7, 25; Nep. Dion, 1, 1; cf.: quid est aetas hominis, nisi memoria rerum veterum cum superiorum aetate contexitur, Cic. Or. 34, 120.—
But “previous”, “former” fits the meaning better here. Note that in sec. 26 of de Sen. the meaning is “previous”:
sed videtis, ut senectus non modo languida atque iners non sit, verum etiam sit operosa et semper agens aliquid et moliens, tale scilicet, quale cuiusque studium in superiore vita fuit.
Again, sec. 76:
sunt etiam eius aetatis: ne ea quidem quaeruntur in senectute. sunt extrema quaedam studia senectutis: ergo, ut superiorum aetatum studia occidunt, sic occidunt etiam senectutis; quod cum evenit, satietas vitae tempus maturum mortis affert.
In both of these passages, senectus is contrasted with superior vita, superiorum aetatum. I think this clinches it: superior aetas in sec. 62, the passage under consideration, must mean “previous life”, not “the last age of life.”
extremos probably means “last, final, in the last stage of life”.