Donald Trump's campaign will likely costs the Republican Party many young voters, potentially for an entire generation. However, young people supporting Democratic candidates is not completely Trump's fault. Many young people are dependent on government to exists, because they are not participating in the work force like previous generations. Therefore, the message of self-reliance that many Republicans support will not resonate with young people.
Of course, Trump's nomination has an effect on future elections. Every primary and every election has an impact on the future. However, I don't believe that Trump's nomination is what will hurt the GOP as far as younger voters go. These same voters do not really support Hillary either. They want a better option.
Donald Trump's nomination will hurt the GOP in future presidential elections. He has exposed vast rifts in the party that will take years to overcome. The conservative in the party will fight to hold on, hurting their prospects with Trump's core voting block. Trump has also damaged the Republican brand through his racist comments.
People vote more with their emotions than their reasoning. If a Democrat wins the 2016 election, but cannot (for whatever reason) satisfy the electorate, then the Republicans will have a decent shot at winning the 2020 election. Well, as long as it isn't Trump 2020. Eight or twelve years from now, folks will simply look back at Trump as a joke (unless he wins!) and won't really let it affect their vote.