I haven't seen this movie, but when I saw the casting I couldn't believe my eyes. Maria Bello is only 9 and 12 years older than those in question, and I could easily accept her as a lot younger than her real age. Tim McGraw also.
shareFlicka was filmed around April of 2005, so Mcgraw & Bello (the parents) turned 38 during filming. Kwanten (brother) was 28, and Lohman was 25 during filming.
The film is set in the summer after what would have been the brother's freshman year in college (if he had gone instead of taking a year to give the ranch a try), so 28-year-old Kwanten was playing a 19-year-old.
I would guess that Lohman had just failed her junior year in high school (but it may have been sophomore) so 25-year-old Lohman was playing a 17 (or 16) year-old.
If we're not supposed to believe the parents are older than the actors playing them, then they had their son when they were 19 themselves.
On my DVD cover it says that Katy is 16 years old.
I am not saying anything about the age difference. I haven't seen the film, but Alison Lohman usually acts very well in every role she has. And acting is something you do when you portray someone that is not your age. Almost all actors do that in movies.