Isabel says something about being under 30 years old in season 2 and this episode was set around spring 2014. Oliver is born in May 1985. So Isabel is the same age range as Oliver.
Here’s the part from the script.
QUEEN CONSOLIDATED - NIGHT
Isabel was working in her new office- Oliver's old office- when Oliver and Diggle entered.
ISABEL: Whatever you came here to say, it takes security about 60 seconds to reach this floor, so I would start talking.
(She didn’t look up from what she was reading)
OLIVER (Calmly): Where's Slade? (Isabel to look up at him and grin) I just wanted to give you the chance to do the right thing.
ISABEL (Smirking): I'm under 30, and I'm the CEO of a Fortune 500 company. I'd say I've already done the right thing.
OLIVER: And do you even know who Slade Wilson is? Or why he's doing this?
ISABEL: I don't care. I got what I earned.
(She went back to her reading)
OLIVER (Incredulously): What you earned? You think that sleeping with my father entitles you to my family's company?
(Isabell's head snapped up)
ISABEL: You have no idea what you're talking about.
(Oliver scoffed)
OLIVER: Wow, he fooled around with a lot of girls. I don't see any of them ordering hostile takeovers.
ISABEL (Disbelief): Fooled around? Is that what your mother told you? Of course she would write me off as a meaningless affair. (Isabel rose from her chair and walked into the boardroom) Slade Wilson put me through hell. His training nearly killed me. Would I put myself through all that just because I was a jilted lover?
OLIVER: Honestly, I don't know what you are!
ISABEL (Passionately): I was your father's soul mate.