Adults who aren't as fit or flexible, or overweight, should choose skiing over snowboarding as they are less likely to have a high impact fall.
On the negative side, skiing can be more taxing on the knees, particularly for someone in their 40s or 50.
Can you learn to snowboard after 50? Absolutely as long as you have some fitness, are willing to take some falls and can think like a kid. When a child heads out on the slopes they go with an open mind and fewer worries. They're not so concerned with making mistakes or looking like a dick.