Countable elementary free groups

We modify the notion of a Fraisse 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 Fraisse limits. We rediscover Lyndon's $\mathbb{Z}[t]$-exponential completions of countable torsion-free CSA groups, as Fraisse limits with respect to extensions of centralizers.