  We present Hubble Space Telescope blue spectra at intermediate spectral
resolution for the nuclei of 23 nearby disk galaxies. These objects were
selected to have nebular emission in their nuclei, and span a range of
emission-line classifications as well as Hubble types. In this paper we focus
on the stellar population as revealed by the continuum spectral energy
distribution measured within the central 0."13 (~8pc) of these galaxies. The
data were modeled with linear combinations of single-age stellar population
synthesis models. The large majority (~80%) of the surveyed nuclei have spectra
whose features are consistent with a predominantly old (>5x10^9 yr) stellar
population. Approximately 25% of these nuclei show evidence of a component with
age younger than 1 Gyr, with the incidence of these stars related to the
nebular classification. Successful model fits imply an average reddening
corresponding to A_V~0.4 mag and stellar metallicity of (1--2.5) Z_sun. We
discuss the implications of these results for the understanding of star
formation history in the environment of quiescent and active supermassive black
holes. Our findings reinforce the picture wherein Seyfert nuclei and the
majority of low-ionization nuclear emission-line regions (LINERs) are
predominantly accretion-powered, and suggest that much of the central star
formation in Hii nuclei is actually circumnuclear.
