DENVER, Feb 13 (Reuters) - Frontier Airlines Inc.(FRNT) on Wednesday said it would retire two Boeing Co. (BA) leased planes early, part of the low-cost airline's move to have a younger fleet. The Denver-based company, which is struggling through a post-Sept. 11 fall-off in travel along with the rest of the industry, said it would take a fourth-quarter charge of $3.1 million, after taxes, related to the lease changes. Frontier said it had amended the leases on two 119-seat Boeing 737-200A aircraft, allowing it to return the planes 22 months before the original termination dates. The airline plans to have three 737-200s left in its fleet at the end of 2003. Frontier's shares closed at $21.61 on the New York Stock Exchange on Wednesday, off 52-week highs of $26.17 set on Feb. 21, 2001.