they will all grow old

The word "they" refers to "the earth" and "the heavens."

your years will have no end

Here David describes the length of time that God is alive as his "years." This can be written in positive form. AT: "you will live forever" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-doublenegatives)