These bikes go through a 'maintenance cycle', and 20-25 years seems to be the time frame that a lot goes wrong, usually seals, plastic parts, and degraded lubrication at moving parts.
This is regardless, as to how many miles the bikes has accumulated,.
Exposure to heat, like we have here in the desert southwest US, also speeds up this process, metal parts aren't effected, due to the dry climate, but everything else succumbs to the relentlees sun and hest.