What is with the timeline?

So, this movie starts in 1755- the year of the Battle of the Monongahela (aka Braddock's Defeat) when Matt Howard is about 10-12 years old. The movie then fast-forwards 12 yrs to assumingly 1767. It ends in 1781- the year of the Battle of the Yorktown. In between 1767 and film's end, Matt marries and has three children.

Yet, by film's end his oldest child is stated to be 18. Also, Matt, his wife and children age remarkably fast between 1775 and 1781. It appears that studio hoped the audience would be either ignorant as to the timeline discrepancies and/or willingly suspend its disbelief.

Does the novel upon which the movie is based also feature the above time-line? My guess is the movie seriously condensed things. Does the novel start in 1755? Did the Howards marry after 1767? What year is Peyton Howard actually born in the novel?