Why does Samuel Taylor Coleridge consider himself as an unwelcome visitor?

In "Youth and Age" I'm thinking that the unwelcome visitor is old age. This is never stated explicitly but there is subtext to support it. Much of the poem is an ode to youth, "Life went a-maying With NATURE, HOPE, and POESY, I was young!" Then the poet reflects on old age which has come. Is it youth in disguise or an unwelcome visitor,

"What strange Disguise hast now put on,

To make believe, that thou art gone?

I see these Locks in silvery slips,

This drooping Gait..."

That's my take on your question for this poem.