We don't know the time when does bitcoins could last. As bitcoin price continue to increase, many people will attract and push them to start earning bitcoin. And maybe after 20 to 30 years, there will a lack of bitcoins but it is just my opinion.
Ofcourse no one can predict the age of bitcoin,but the value of the bitcoin is constantly increasing these times as well as sometimes the value of bitcoin falls little bit.when we research about the bitcoin statistics,it is doing much more better.As you mentioned there maybe lack of bitcoins in the future .so be wise and act immediately investment on the bitcoin will surely gonna be a better Investment in the future.