And, darling, I will be loving you 'til we're 70
And, baby, my heart could still fall as hard at 23
And I'm thinking 'bout how people fall in love in mysterious ways
Maybe just the touch of a hand
Well, me—I fall in love with you every single day
And I just wanna tell you I am

The second line of the stanza contains a simile, "still fall as hard at 23." He is describing how he will feel the same way from 23 until 70 (young to old).