The Lunar Radar Sounder (LRS) performed global soundings and detected possible subsurface echoes at the nearside maria. This paper examines the relationship between the distribution of the detected subsurface echoes and the surface ages in the western nearside maria. Continuous subsurface reflectors are clearly detected only in a few areas consisting of about 10% of the western nearside maria at apparent depths from hundreds to more than a thousand meters. These reflectors are not generally the basements of the mare but are the interface between different basaltic rock facies. Comparison between the distribution of the detected subsurface boundaries and the surface ages suggests that most of the detected subsurface reflectors were formed more than 3.4 billion years ago. Based on the strong connection between the accumulation rate of regolith and the absolute age, the detected subsurface boundaries appear to be relatively thick regolith layers accumulated during the depositional hiatuses of basaltic lavas.