First off I am not a doctor. But, when I consulted my own and my children's pediatrician, they said after 1.5 years is good to go. They have developed sufficient immunities and gotten all their current shots. I was bit worried when I took my first child to Turkey, I checked the CDC website (www.CDC.gov) for diseases and other fun. Check my links and you can easily access the CDC and check the area of the world you are traveling to.
But, I found no problems with my kids being 1.5 for our first trip to England, Italy, Turkey, Greece, and various Greecian Isles.
You will find that people can be quite thoughtful when traveling with smaller children. I will never forget Paris and the disgruntled Metro (subway) employee. When after being salty to other people he looked at youngest at the time who was 1.5 and helped graciously helped me out. This is even after I pulled the ignorant american, I did not know where we were going. Even though I had been to Europe 10-12 times, I had never been to France. That is another story.
Traveling with Children can be quite a positive thing. You will find that people can be quite open.