Looking for the oldest serving electricity pylons, I found Necaxa - Mexico - El Oro from 1905 is still partly in service between the Necaxa power station and Tulancingo, as well the branch line to Pachuca and a small section around Ixtlahuaca de Rayón. These towers are identical to the towers of the Niagara - Toronto power line from same year.

https://reference.insulators.info/publications/view/?id=3414