The fourth Doctor was aged in 'The Leisure Hive', and the tenth in 'Last Of The Time Lords'. Admittedly, both occasions were the result of technology rather than natural ageing, but they do show the process occurs.
I'd speculate that Time Lords have a certain degree of control over the ageing process. Free from distractions and harmful influences a Time Lord could slow, or perhaps even hold in check, natural ageing. But a busy, eventful life might allow it to proceed as it does for other species.