“I know but too well how time flies along, That we live but few years, and yet fewer are young.”
The speaker recognizes that both life and youth are brief, then uses that awareness to explain why a relationship should be worth the time and consequences it demands.
Source details
The Lover: A Ballad, represented by the University of Toronto Representative Poetry Online text from Robert Dodsley’s 1748 A Collection of Poems by Several Hands. The line belongs to a lyric speaker weighing desire against the risk of later regret; it is not treated as a verbatim autobiographical confession by Montagu.