18 has a representation as a sum of two squares: 18 = 32 + 32.
18 = 9 + 9 and 81 = 9 x 9.
18 is the only number that is twice the sum of its digits: 18 = 2(1 + 8).
The cube together with the 4th power of 18 use all the digits 0 to 9 once each: 183 = 5832 and 184 = 104,976.
18 is the smallest number with two representations as a sum of three primes: 18 = 2 + 3 + 13 = 2 + 5 + 11.
18 is the number of integer partitions of 13 into distinct parts.
18 is 33 in base 5.
18 is the voting age in many countries.
Source: Number Gossip