Anatomical analyses of occipital and temporal cortex of patients with fragile X mental retardation syndrome (FXS) and in a mouse model of the syndrome (FraX mice) compared to controls have suggested that the fragile X mental retardation protein (FMRP) is important for normal spine structural maturation and pruning. However, a recent analysis of spine properties in somatosensory cortex of young FraX mice has suggested that this region may not exhibit spine abnormalities. While spine abnormalities were present 1 week after birth in somatosensory cortex, by 4 weeks almost all spine abnormalities had disappeared, suggesting that adult spine abnormalities observed in other cortical regions may not persist post-developmentally in somatosensory cortex. To resolve this discrepancy we examined spine properties in somatosensory cortex of young (day 25) and adult (day 73-76) FraX compared to wild-type (WT) mice. Spine properties in young FraX and WT mice did not consistently differ from each other, consistent with the recent analysis of developing somatosensory cortex. However, adult FraX mice exhibited increased spine density, longer spines, more spines with an immature-appearing structure, fewer shorter spines, and fewer spines with a mature structure, a pattern consistent with prior analyses from other adult cortical brain regions in humans and mice. These findings (1) support the previous report of the absence of major spine abnormalities in the fourth postnatal week, (2) demonstrate normal spine development in WT mice, (3) demonstrate abnormal spine development after the fourth postnatal week in FraX mice, and (4) demonstrate spine abnormalities in somatosensory cortex of adult FraX compared to adult WT mice. In doing so, these findings resolve a potential conflict in the literature and more thoroughly describe the role of FMRP in spine development.