Kevin
As far as I know, age does not affect the speed of training. Increasing the level of your Simulator building will increase speed of training and having a high level Doctor will reduce health loss when you train thus enabling you to do more training each day.
Apart from the above, the thing that most influences training speed is star rating (driver level relative to team level). The more stars the driver has the slower the training will be.
Have a look in the guides for optimum training strategy, but rule of thumb is to maximise physical attributes (stamina and green BMI), then driving abilities before finally focusing on the mental stats.
thanks a lot