The first reported sightings occurred right after the Krakatoa eruption of 1885. For a while they were reported near the poles but recently the clouds have been sited further and further away from the poles. Some have made the claim the clouds' advancement is tied with climate change.
The proof for this is lacking in amount. Krakatoa caused the end of the initial wave of warming after the Little Ice Age. However, no change in amount or position seems to have been noticed during the cooling spells of the early 1900s and 1940 to 1975 trend. Same goes for the warming trends between those periods. Only recently have the noctilucent clouds been noticed so far away from the poles.
For more information on Noctilucent clouds there is the NLC Observers' Homepage were one can report and view reports of the clouds. There is also NASA's Noctilucent Clouds in the Classroom which offers resources to learn and teach with.