ט אַל-תַּשְׁלִיכֵנִי, לְעֵת זִקְנָה; כִּכְלוֹת כֹּחִי, אַל-תַּעַזְבֵנִי
9 Cast me not off in the time of old age; when my strength faileth,
forsake me not.
י כִּי-אָמְרוּ אוֹיְבַי לִי; וְשֹׁמְרֵי נַפְשִׁי, נוֹעֲצוּ
- For mine enemies speak concerning me, and they that watch for my soul take counsel together,
יא לֵאמֹר, אֱלֹהִים עֲזָבוֹ; רִדְפוּ וְתִפְשׂוּהוּ, כִּי-אֵין
- Saying: 'God hath forsaken him; pursue and take him; for there is none to deliver.'
This is Tehillim 71:9 Rashi says (my translation from Mikraos Gedolos)
If I become old with sins, that is to say, I do many sins.
Metzudas David says (my translation)
So that "they" shall not say that everything that I have accomplished
in the past was because of my strength and Hashem did not do all of
this and now that there is no more in my strength, in the time of old
age, because my strength has ebbed.
It appears that this means that even though I have not atoned for all the sins that I have done during my lifetime or that the rewards that Hashem has granted me during my life exceeded that which I deserved, please do not punish me for my aveiros now. See the next few verses in which he says that his enemies would blame the misfortunes on there being no Hashem ("none to deliver")