Intellectual impairment in addition to memory problems in temporal lobe epilepsy (TLE) has been postulated to be a result of a mental decline caused by chronic epilepsy. Longer duration, however, is often confounded with earlier age at onset of epilepsy (AOE). IQ and memory were evaluated in 188 patients (age>16) with TLE with an AOE before age 14 (N=91) or after age 15 (N=97). Earlier AOE did not differentially affect memory aspects more associated with temporomesial structures, but it negatively affected IQ and aspects of memory that are more related to temporolateral and extratemporal lobe structures. As earlier AOE was also associated with lower educational levels and because one may not assume that these are lost with chronicity of epilepsy, the results can be interpreted as reflecting developmental hindrance of extratemporal lobe functions in early-onset TLE rather than cognitive decline due to a longer duration of epilepsy.