It's hard to say what is going on without more info, as freemike mentioned, fill out the sticky and post on here so we can see if there may be another underlying issue with his behavior.
Each betta will be different as they age, but typical signs of age is dulling/muting of the color (not to be confused with a sick betta who has become pale), so that the color isn't as bright as it once was. That can happen subtly, or quite significantly over time.
Also they tend to start getting a humpback.. in a sense can look like a salmon.
They can become "bony" in appearance.. and individual scales may look more prominent.
Loss of sight is quite common- not being able to see well, cataracts that look like a white film over the eyes. You can tell if losing sight by him lunging at food and missing, not noticing things outside the tank as often, etc.
Gradual slow in activities.. older bettas tend to lay around most of the day, whereas young ones should be quite active. But it is gradual, if it seems sudden then there is most likely something else going on with your guy.
Older bettas may not show the enthusiasm for food like they used to. They may show less interest in their humans or stop their “wiggle dances” entirely. Their tenacity for flaring may lessen over time as well. A betta that used to put on a show for other males or his own reflection may not display with the gusto he once did.
All of those signs should be gradual, to where you don't notice them happen "all of a sudden"... if one day he is fine, and the next he is showing these sort of signs then that means there is something else wrong with him that isn't age related.
Without more info on care then it's hard to say what is going on. Normally staying at the top like that could be an indicator of water quality issues rather then age.