i was wrong...it is 21 yo. installed in 1992..... i miscounted the millenium transition years
..... this is its 22nd year of use.
i have no idea what has made it last so long... i burned wood in it along with coal in the early years so maybe there is a microscopic layer of creosote protecting it against the coal ash reaction. although i never had any creosote build up in the chimney in all the years i burned wood along with the coal, whenever i would brush it out it was mostly clean except for a thin layer of ash.
I do the same thing every year, disconnect the inside black pipe and plug the thimble, brush it out at the end of the season, and i take the cap off the bottom of the chimney so it is open from bottom to top for air to flow through. It has a cap on it, don't know if that helps or not. Even the bottom cap that lays in the fly ash residue all summer at the bottom of the chimney is still solid.