Here comes the frustrating part, Dungeons and Dragons monster/race lore is fairly liquid in nature. Usually things stay canon until mentioned otherwise, but the community as a whole kinda plays telephone based on half remembered readings from the Forgotten Realms wiki. With regards to Mind Flayer/Illithid aging, Volo's Guide to Monsters (2016) is a Fifth Edition book that mentions that Mind Flayers that are aging, infirm, or grievously injured are incorporated into an Elder Brain. However, it makes no mentions of how old a mind flayer has to be to die of old age. For that the most common citation points to the Monstrous Manual (1993) which is for Second Edition and under TSR rather than WotC. It says that Mind Flayers have a lifespan of 125 years. There might be some more information in some Third and Fourth edition, but ultimately the dilemma ends up being: Which source do we trust?
