This wrong, no matter how old a coin is as long as there are investors in the coin and people willing to put their money in it then they can make you money. It is just a matter of time just like other coins and with old coins you can be more sure that they are less likely to die out when you are holding the coins for the long term
The guarantee of safe investment in both old and new coins is vice versa because some old coin are not good for long term investment same as some new coin either and if a coin still have some investors who still put money in it doesn't mean it will yield a good return.