I don't think the script writers were consistent with Laura's age throughout the series, and I don't think she aged at the same pace as the show. In "To See the World", Johnny Johnson says that Laura is 8 years old. In "The Music Box", Laura has a birthday, but it's not specified which one. In "Sweet Sixteen", which is at the end of season 6, Laura turns 16, which would make her 11, not 8, at the end of season 1 if she had aged at the same pace as the show. In the last three seasons and the movies, I think Laura is meant to have aged well into her twenties, even though other characters, like Nancy, have aged at the pace of the show.
Melissa Gilbert herself was, I think, 9 when the pilot movie filmed and 19 when "The Last Farewell" filmed.