I would certainly wait till your 26 Lee, if thats possible, as it opens up the entire scheme to you. Extending your lease to 4 or 5 years is an option but the criteria changes every now and again. In our case we were allowed to extend to 5 years as we had paid more than £2.000 AP. In 3 years time this could have changed.
Motability wise All vehicles become available on your 25th Birthday