Granted, 'Aliens' was almost 30 years ago but I believe both actors could pass for a 10 year gap on film. As has been stated here - Biehn could have latex make up FX to his face. I recently saw Sigourney Weaver on the Jonathan Ross Show in the UK (skip to 0.15 in the clip), and I was surprised at how good she looked for 65 years old. Obviously the make up and lighting of a TV studio helps. Personal trainers and nutritionists should help them lower any age puffiness/weight for a 3 month shoot too.
The characters have also gone through some serious traumatic stress in their lives, and would be susceptible to age more quickly.
Biehn and Weaver being 30 years older doesn't seem like a huge issue to me. Although if this was to be set between Aliens and Alien 3, then yes, creditability would be stretched.