Some people think they are immune to poison ivy because they have never had a rash. However, it is possible to eventually break out into a rash after being exposed to poison ivy or oak enough times.
You can begin getting a rash at any time during your life. Like other topical allergies, sensitivity is just a matter of being exposed enough times until the body has an allergic reaction to the rash-causing oil, urushiol. Since three out of four people are sensitive to urushiol, it is only a matter of enough exposure before most people react to the plants.
Sensitivity may also increase over time. With repeated exposure to the plants, you may find the rash is more severe. Once you know you are sensitive to urushiol, it is important to remove the oil as quickly as possible when you know you have been in contact with poison ivy or oak.