What 'agent'? Do any of these 'agents' know more about this?
I seriously doubt they will attempt to audit all foreigners here 20 years back or anything.
Getting an 'agent' might be easier to do in Bangkok, Pattaya, Phuket, Chiang Mai, Hua Hin - other places? Lucky to find someone with good English. Not seeing the point of paying an 'agent' now, for services unspecified.
For those of us not stinking rich, and countries having tax agreements, the yearly tax on the income might not be such a big deal, or not worth the hassle of getting an 'agent' etc.
if it's not too much money, and no easy way to avoid it, guess I'll just grumble and pay up.
If it's a large sum, a paperwork nightmare, or some sky-is-falling situation, reconsider.