While age effects in reaction time (RT) tasks across the lifespan are well established for level of performance, analogous findings have started appearing also for indicators of intra-individual variability (IIV). Children are not only slower, but also display more variability than younger adults in RT. Yet, little is known about potential moderating sex effects on RT-IIV. We analyzed responses in a simple RT task with 120 trials in children, younger, and older adults. To best capture sex differences we used generalized additive models (GAMs), a semi-parametric regression approach, to fit splines relating nonlinearly age to RT, and capable of testing sex differences therein. This method is more adequate to test sex differences in nonlinear age relations than polynomial regression. Results show that (a) males are faster than females (except in the older adults), and (b) in younger and older adults, males are less variable than females. No sex difference in IIV emerged in children. Results are consistent with the hypothesis that sex differences in RT variability may be attributable to brain effects of sex hormones, in particular estrogen, whose receptors are present in several brain regions involved in information processing and attention, which are systems involved in the regulation of variability in information processing. Thus, according to this hypothesis, sex differences in RT-IIV should be present after puberty, but not in pre-pubertal children.