The poet in Campbell's poem explains,

'Tis the sunset of life gives me mystical lore.

Outside of Scotch poetry one finds, occasionally, a similar faith in the
old poet. Mrs. Browning's observation tells her that maturity alone can
express itself with youthful freshness. Aurora declares,

I count it strange and hard to understand
That nearly all young poets should write old.
... It may be perhaps
Such have not settled long and deep enough
In trance to attain to clairvoyance, and still
The memory mixes with the vision, spoils
And works it turbid. Or perhaps again
In order to discover the Muse Sphinx
The melancholy desert must sweep around
Behind you as before.

Aurora feels, indeed, that the poet's gift is not proved till age. She
sighs, remembering her own youth,

Alas, near all the birds
Will sing at dawn,--and yet we do not take
The chaffering swallow for the holy lark.

Coinciding with this feeling is Rossetti's sentiment:
... Many men are poets in their youth,
But for one sweet-strung soul the wires prolong
Even through all age the indomitable song.