Once the villagers become "elderly", they slow down -- this means they perform tasks slower (so they don't succeed as often as they did when they were younger).

Build up your medicine tech, because it keeps the villagers from being elderly until they are a bit older, and allows them to live longer (65 is a bit young - I bet that villager would have made elder if they had gotten to age 70).

I think it also becomes harder to learn as they get more skill, which makes a lot of sense -- which means that even though they are successful at the skill, they don't always gain skill.

I like Arnie's strategy -- use the youngest villagers to farm and build, and then put them on the research table when they get older. It's actually easier once you have a few more villagers, since you can have several villagers researching and farming to get you tech and food.

Good luck!