Fra\"iss\'e limits of limit groups

We modify the notion of a Fra\"iss\'e class and show that various interesting classes of groups, notably the class of nonabelian limit groups and the class of finitely generated elementary free groups, admit Fra\"iss\'e limits. Furthermore, we rediscover Lyndon's $\Z[t]$-exponential completions of countable torsion-free CSA groups, as Fra\"iss\'e limits with respect to extensions of centralizers. Dedicated to the memory of Charles Sims.