Can someone in their 20s have an aortic aneurysm growing in them already?

Yes: Though rare, there are certain genetic defects which cause a weakening in the walls of arteries including the aorta which can lead to aneurysms in younger patients.

Very unusual : Typically aortic aneurysm occur later in life after 50's but can occur in younger patients especially with some connective tissue disorders like Marfan or ehler danlos syndromes . Infected or inflammatory aneurysms can also occur but rare.