Rashi citing Avos 3:1: Look at (ponder) three things (from where you came, to where you are going, and in front of Whom you will need to give reckoning, and you will not come to sin. Yerushalmi Sotah 2:2 - we expound all from our verse. "Bor'echa" is your Creator,- in front of Whom you will need to give reckoning. We read this also like Be'ercha (your wellspring, from where you came, i.e. semen) and Borcha (your pit (grave), a place of earth and worms).

Rav Sadya Gaon: Strive in memory of your Creator [Seforno - in investigation and deed, to honor Him].

Rashbam: Refrain from sin, and repent [before "Yemei ha'Ra'ah"].

Rashi (6, from Eichah Rabah Pesichah 23): This is while your Bechuros (chosen ones) are intact. While there are Kohanim - "u'Vachor Oso mi'Kol Shivtei Yisrael Li l'Chohen" (Shmuel I, 2:28). While there are Leviyim - "Ki Vo Vachar Hashem Elokecha mi'Kol Shevatecha" (Devarim 18:5). While Malchus Beis David exists - "va'Yivchar b'David Avdo" (Tehilim 78:70). While Yerushalayim exists - "ha'Ir Asher Bacharti Vo" (Melachim I, 11:32). While the Beis ha'Mikdash exists - "v'Atah Bacharti v'Hikdashti Es ha'Bayis ha'Zeh" (Divrei ha'Yamim II, 7:16). While you are alive - "u'Vecha Bachar Hashem" (Devarim 7:6).

Seforno: It is when you can despise evil and choose good - but you could also do oppositely, like Chazal said 'do [Tzedakah] while you find [who to give to] and have [what to give] and while it is in your power' (Shabbos 151b).

Metzudas David: It is when you have the ability to go in Hashem's ways and serve Him.

Rashi citing Shabbos 151b: It is old age ¹ . Ibn Ezra - or, days of a mortal illness, even for one who is not old.

Rashi (6, from Eichah Rabah): It is Galus.

Seforno: It is loss of strength in old age. Metzudas David - then, it is much harder than in youth.

Torah Temimah citing Shabbos 151b: They are Yemos ha'Mashi'ach, when there will not be merit or sin (Rashi 151b - for giving or refusing to give Tzedakah, for everyone will be rich ¹ . Ramban (Devarim 30:6) - there will be no desire to sin.)

Rashbam: You will say that you have no desire for your early deeds, because you do not know the day of your death, amidst weakness in your old age.

Seforno: I have no desire for any pleasure of this world. Then it is not such a merit if you do not sin - when a thief cannot steal, he considers himself upright (Sanhedrin 22a, according to Rashi there).

Metzudas David: You have no desire; you will prefer death to life.