I’m not so sure it’s his age as much as his looks. He looks younger than he is. I doubt Trump actually knew how old Rubio is when he said that.
As for his knowledge of Syria and foreign policy, Rubio is on the Senate Committee on Foreign Relations and Senate Select Committee on Intelligence.
I have never heard him say anything on foreign affairs that I didn’t think was accurate and I agree with probably 95% of it.
If you had ever sat in a committee hearing you wouldn’t be so quick to assume that Rubio is knowledgeable just because he’s on Foreign Relations and the SSCI. I had to brief Nancy Pelosi (sans staffers) on some classified issues when she was the Ranking Member of the HPSCI (House Intelligence Committee) and I was shocked at how little she understood what I was briefing her on.
Members of the House and Senate - even the “good” ones - are generalists, not experts. With VERY few exceptions, the brains in the committees are the senior staffers, not the members. Staffers write the questions and coach members through hearings. I’ll admit that Rubio is articulate and very well prepared, but I strongly suspect that some more specific and more focused questions would trip him up badly.