It's pretty hard to tell the age of an adult chicken once they reach sexual maturity, but I agree you probably are in the ball park.
There are signs that can indicate if it is younger or older.
Think back to when he showed up...was his plummage as full as it is now? Did he fill out in muscle mass since then?
Roosters often don't come into their full plummage or body weight until they are a year of age, and not into their full colors until after their first adult molt at 18 months.
Are his legs smooth without a lot of scaling? While scaly leg mite can cause rough legs in young birds, it tends to creep in with older birds as their immune system runs down a bit. Rooster spurs also continue to grow as they age. Were his little nubs when he showed up and now longer? Spur growth varies a lot with genetics, but many do not begin to grow a spur until 8 or 9 months.
Overall, younger birds simply look fuller, plumper, shinier plummage, brighter combs, crisper eyes, sleek legs.
All of that can be skewed with poor health and maintenance.
LofMc
