Above 70 years old you do get a free lift pass, you claim them the same way as the discounted ones I mentioned above. The transfer details and times are here The bus company will send buses suitable for the number of bookings they have. whether thats a mini bus (never normally) or 3 coaches, depends what they need to cover their booking numbers.
65-69 year olds get a discounted lift pass at €23 per day for the whole Grandvalira area. It's not available to book in advance as you have to take your passort to the lift pass kiosks at the bottom of the slopes to prove your age and be able to purchase it. But.... when you do eventually hit 70 come back again because then it's FREE!!!