A good example would be Star Trek the undiscovered country. It was filmed in 1990 and released in 1991. At the time of filming, the three main leads William Shatner, Deforest Kelley, and Lenard Nimoy were 60,71,60. Ford will be 71 when filming starts next year,Fisher will be 57, and Hamill will be 62. So it can be done...will they be in all three? If so the final movie will be in 2019 or 2020....Ford will be 76 or 77 years old.