Suffered a engine fire iirc.
To be fair, vehicles reaching that age were perhaps less frequent than you may think. The slowdown in fleet replacement around deregulation resulted in some vehicles lasting longer but in the 1970s, Glasgow were disposing of early PDR Atlanteans at the 14 year mark.
There's not as many ALXs and Presidents requiring replacement about as you'd think. First West Yorkshire have only about 20, and South Yorkshire have got rid of all of theirs, whilst Bristol is down to penny numbers.
The requirement for social distancing etc meant that quite a few were retained for longer than anticipated so whilst the South West has quite a number, many ALXs are heading to Ensign fairly imminently and the Presidents are on borrowed time.
Aye, when I think of motors that have achieved 18+ years of service in Glasgow service (at least with the Corporation and it's successors), those that spring to mind most readily are a few Atlanteans (those reinstated post-Larkfield fire in particular, not sure if LA664 was one of them), a few Ailsas (the last ones were between 20-25 years old when they finally went in the summer of 2006), the last ex-Central Tigers, the last 51-reg B10BLEs out in Dumbarton/Vale of Leven (withdrawn last March; at least two linger on as driver/route trainers) and most recently, Presidents 33120 & 32300/301 & 303-305 (they turned 18 this past May; 32302 didn't quite make it). There are no doubt other examples, but those are the ones that spring most readily to mind.
As you rightly say, social distancing etc is the primary reason Glasgow's (and other opcos where applicable) last Tridents have lasted as long as they have, same goes for the President-bodied B7TLs (one has been withdrawn though, namely 32302 as above). As has been discussed here at length, I suspect - am willing to be proved wrong as ever though! - the cascade the arrival of the EVs will trigger will likely see them all off as a group, heading either for scrap, non-PSV use or even preservation.