Plug-In Electric Vehicles, or PEVs, represent a new load type characterized by a significant magnitude and duration. Potential impacts to distribution service assets in light of customer adoption of PEVs are therefore a natural concern. One particular issue is the degree to which PEV demands - charging during peak demand hours as well as during off peak hours through controlled charging - will result in additional thermal insulation degradation across utility distribution transformer assets. However, thermal aging of transformer insulation is a complex issue involving many factors including transformer thermal characteristics, ambient operating conditions, as well as total load characteristics over time. The nonlinear nature of the thermal aging also indicates PEV impacts to be condition specific; therefore, thermal aging due to PEV charging must be evaluated for each specific condition set. The influence of PEV charging demands on transformer insulation aging is evaluated in this paper through a series of sensitivity analyses. Results are derived via specific case simulations and evaluations using ANSI/IEEE standard C57.91-1995 assuming possible PEV charging scenarios likely to be experienced by the North American utilities. Qualitative results and conclusions are derived addressing the key issues and behaviors intended to highlight the conditions under which detailed transformer insulation loss of life studies may be required.