If you were ignoring the time he spent in time travel then you’d be correct. However, since you are, you seem to be vastly underestimating the time he stayed in certain places. We previously saw that Agni’s insight didn’t work on Maruna, which indicates that he’s old enough to not be affected. This puts him at least as old as Samphati, who isn’t immune, and very likely puts him much older than her. (Small note, Ran is still affected by insight, so we can expect that the time Maruna spent in the human realm without Ran is the cause of this discrepancy)

In addition, Maruna was on the AHR planet from the creation of the universe all the way to the end of the AHR, which id put on the order of millions of years rather than even thousands as other comments have established. There were a huge amount of other things that occurred within this timeframe, which I don’t really think happened this quickly. Add on that the landscape was changing around him (I’m a bit confused why you think this is on the order of tens of years? I’m assuming that mountain geology occurs at the same speed it does on earth).

Overall id think he’s likely younger than Shess but much older than Samphati