Yes you can say that it is too late to invest in Bitcoin because the price is 18,000$ which is not a small amount.It is not affordable by everyone .Only the people who have enough money can invest in it .Other people may arrange the money from somewhere to invest in it because they will earn enough profit in small period of time to return the amount of money they borrowed .
Even so,if he or someone else would like to invest inside of it,then they can also do it,for the reason that one of the main characteristics is that it is very divisible although it is true as you just mentioned above,the price of a single BTC is very high but after what I just mentioned,even so, it is possible that if someone decides to invest they can do it,however,the decision or responsibility to do it or not do it depends completely on each user or investor.