Newt will be 73 in about a month, 3 days after Trump hits 70.
Besides the age issue, Newt negates the effectiveness of some things that Trump was already hitting Hillary on concerning her husband.
But Newt is obviously a very smart guy and in many ways seems like a strong pick.
Yet Trump needs to win over voters who think he is too mercurial or might do something overly extreme. Reagan once said in 1980: "I’ve got to convince people I’m not a cross between Ebenezer Scrooge and the Mad Bomber." Biden helped Obama with this kind of thing in 2008 because he seemed more down to earth. It's not clear that Newt does this for Trump besides his obviously having a lot more experience with government.
In the past Newt has sometimes said things that came across as insincere. There are some similarities between Gingrich and another politician who has been in the news lately.
I am neutral on Newt but I think Trump will go with a woman/minority.