Of course there's no way, hence why I said it's an assumption. But yea, I misunderstood the question a bit. 
Initially misunderstood it as "people who held bitcoin 5 years ago". Whoops.
Initially misunderstood it as "people who held bitcoin 5 years ago". Whoops.While it's impossible to find out how many people who did that, you can get rough estimation how many bitcoin was held for at least 5 years by open blockchain explorer (such as Blockchair) and filter for any UTXO with age older than 5 years.