I understand Madoff's scheme started in 1960's/1970's. The feeders to the fund date way back. The SEC enforcement action in the early 90's wasn't directly related to the ponzi scheme, more aimed at the feeders. The wiki pedia article has this "Federal investigators believe the fraud in the investment management division and advisory division may have begun in the 1970s.[27][failed verification] However, Madoff himself stated his fraudulent activities began in the 1990s.[28] ". I would trust the Feds. https://en.wikipedia.org/wiki/Madoff_investment_scandal