Towering is the confidence of twenty-one.

Samuel Johnson (1709-1784) English writer, lexicographer, critic

Letter (1759-01-09) to Bennet Langton [paraphrase]
    (Source)

While usually quoted as above, the actual passage reads:

When I was as you are now, towering in the confidence of twenty-one, little did I suspect that I should be at forty-nine, what I now am.

Langton was only 14-15 at the time.

The letter is itself misdated (and often mis-cited) as being in 1758 when it was actually written in 1759 (based on Johnson's stated age and other internal evidence).

In James Boswell, The Life of Samuel Johnson, Vol. 1: 1709-1765 (1791).