Hi Ange,

It can always be difficult for older defence to mark young, quick players, however it is important to remember that your older players have experience on their side and they should use this to their advantage. If your GD is not quick enough to go out for the centre pass and get back on the GA, you can make the decision to let her get the first phase centre pass as she is not a threat outside the circle. This will allow your GD to conserve her energy and and get back, as the GA will inevitably be driving down court towards the circle and your GD can pick her up there. 

I would also advise your GD keeping her high and wide where possible, this is high up the court (away from the circle) and wide towards the sidelines to stop her being a threat. As soon as the GA has released the ball get your GD to step in, to mark man on man, and position her body to push her towards the sidelines and prevent her getting down court, this will definitely slow the GA down. 

Ultimately if your GD does lose her tell her to get back to the circle edge and prevent the GA entering, as you know that is where the GA wants to be. 

Using these tips will allow your GD to work smarter, not harder. 

Hope this helps.