Olsson was a great player in his prime, he never had any pace back then it was all about positioning, organising and the physicality so in theory those shouldn't be that diminished by age. He said as much in his interview after joining.
Warnock, Perkins, Jussi, DeZeeuw, Henchoz and Matty Jackson all did a great job for us despite their advancing years so i'd never say the age of a player means they automatically wont be any good. He may be another ever green player who can still perform at this level.
We paid zero money and he is here until the summer, likely on very low wages - so at worst we've got an experienced back up player who will only see first team action if we have an injury crisis and can help the younger players with his experience and leadership, at best we have a good defender who will make us better.
This signing doesn't bother me in the slightest, it's super low risk if it goes wrong with a potential big upside if it goes right. Age means nothing, it's what they do on the pitch that counts.
Fox and Olsson could end up our centre half pairing and being our throw back to the fantastic job DeZeeuw and Henchoz / Jackson did despite their advancing years.