The age difference has been easily explained with how the post-apocalyptic conditions have grizzled Rick.
If there are to be three Walking Dead films to come, it could be interesting to see multiple stages of Rick.
Another thing to consider concerning the delays in production is the real-world/Walking Dead universe time differences.
Andrew Lincoln has been Rick for over ten years our time, but for only about 5 years in Rick Grimes time if the movie takes place directly after his exit helicopter ride.