There are three men to call themselves Ant-Man in the 616 Marvel Universe.
HANK PYM 1ST ANT-MAN, SCOTT LANG 2ND ANT-MAN, ERIC O'GRADY 3RD ANT-MAN
While each guise is interesting and engaging in their own way. Hank Pym is the quintessential bearer of the mantle.
Pym is a truly layered character full of various strengths and weaknesses. Hank is primarily a research scientist in the field of biochemistry. Hank is a renowned physicist (but most importantly and not widely known) with an expertise in the fields of robotics, ANT-MAN's helmet makes that obvious since it was his creation.
Also he is responsible for creating the very first artificial intelligence in the 616 Marvel Universe.
This explains how Ultron can become both Stark and Pym's fault. Stark introduces his A.I tech into SHIELD and THE AVENGERS, but Pym could have introduced the A.I ability to Stark originally. In which case HANK still creates ULTRON inadvertently
In a way they can both be responsible for the AGE OF ULTRON and Pym can still "create" Ultron. It's a slim possibility but, who knows? the rationale makes sense. I encourage everyone to research as well, Wright has said that there are Pym flashbacks in his LANG centric Ant-man film.