You will want to enjoy the entire trek and not feel constantly pushed to your physical limits: it is better to suffer a bit in training than suffer whilst on your holiday. The trek pace should feel like low intensity exercise and you should be able to walk and talk when ascending.
If you take regular walking weekends and are happy covering 16-22km (10-14 miles) each day, with a number of ascents and descents, you will comfortably finish the trek. If you have a sedentary lifestyle and go on short walks once a month you will need to get much fitter in advance. During a typical day on the TMB you should expect to walk for between 6 and 8 hours and be outdoors for 8 or 9 hours. Most of the ascents and descents are not steep, but on 5 days you will climb and descend 1000m.
Age should not be a barrier in any way. Previous clients have ranged in age from 14 to 76 years old. In our experience, the least prepared and unfit are men aged 40-50 years who work in offices.