How about a pic from the side to show how angled the teeth are.
The older they get the harder it is to tell age from the marks on their teeth. So much so that at one point, all horses over the age of 8 were referred to as 'aged'. 'Aged' as in 'over 8', not as in 'real old'. That was the expression.
My dentist says it also depends on how they are kept. One of our horses was locked up in a barn for several years and never let out - so doc says his mouth looks, 'deceptively young'. In other words, if not grazing or otherwise not fed traditionally, that affects all the wear patterns. In that case he takes into account a lot of other factors - slant and general shape of teeth, appearance of the head, body, hair coat, etc.
Since you don't know the horse's history, it might work out best to take in all the clues, but it's still a matter of being content with an estimate.
Generally if a horse is older, the seller always shaves off a few years off the age - if he even knows it accurately. Often with unregistered, untatoo-d horses, no one really knows.