The age of the new person is 19 years old. We can determine this by calculating the total age of all 26 people in the theatre before the new person arrived. The sum of their ages is 26 * 18 = 468. After the new person arrives, the total age becomes 27 * 19 = 513. The age of the new person can then be found by taking the difference between the two totals: 513 - 468 = 45. So the age of the new person is 45 - (26 * 18) = 19.