We investigated strategies used to estimate answers to addition problems. Two hundred and sixteen participants (72 adults, 72 sixth graders, and 72 fourth graders) had to provide estimates of three-by-three digit addition problems (e.g., 249 + 743). The choice/no-choice method was used to obtain unbiased estimates of the performance characteristics of strategies. Results showed that (a) at all ages, the most common strategy was to round both operands down to the closest smaller decades, (b) strategy use and execution were influenced by participants' age, problem features, and relative strategy performance, and (c) age-related changes in computational estimation include changes in relative strategy use and execution, as well as in the relative influences of problem and strategy characteristics on strategy choices. Implications of these findings for understanding age-related differences in strategic aspects of computational estimation performance are discussed.