I hear people say "it is allmost impossible since you need to own 51% of the coins" but is that really the case?

Isn't it that one should own 51% of the current available

**coin age**at any given time?

Lets say we have a coin with these parameters:

Total coins: 100 000 000 (virtually all in circulation)

Min coin age for staking: 24hr

Max coin age: no upper limit

Now, how mutch of these coins are elegible for staking and in wallets that is up and running at any given time?

I have no numbers but lets estimate 20%.

What would the avarage coin age be for these coins at any given time? Estimate 3 days

That gives a total staking-power (coin age * coins) at any given time of 60 000 000

If i transfer 1 000 000 coins and leave them in my wallet offline for 123 days then i will have 51% of the total staking power hence make it possible for me to perform a 51% attack.

As i said, i don't have any real numbers just estimates and they could be way wrong but if the coin doesn't have any coinage upper limit i think this is a real risk...