They would be meaning playing DOtA since age 7. The original DOtA has been played since pre-2005 under various guises. Dota 2 is largely identical to the original dota as far as gameplay goes.

Exactly where DOtA starts and ends can be a little bit fluxy. It has evolved in a rolling fashion over many different patches that often tweak gameplay in large ways.

Maybe they meant Dota 1, which was released in 2005? But that still makes no sense.

They mean the original Dota.