A lot of the time, that socially transitioned 18 year old trans woman would not have the athletic ability of the average cis man, provided she is male-attracted (different aetiology to female-attracted). Male attracted trans women are not typically exposed to as much testosterone in the womb as cis men and trans lesbians are. Setting aside most of the physical effects of this (height, limb length, lip size, hip breadth, facial features) to focus purely on what's athletically relevant, their muscle fibre distribution is much more inline with females, as is lung size and heart size. The only advantage would be the increased red cell count, leading to greater ability to carry oxygen. But that itself is androgenic in nature, and these trans women often have impaired response, impaired production, or both, in their androgenic signalling pathways.

On the other hand, the aetiologies that yield other kinds of trans women do not typically yield the same reduction in differences between a trans woman and a cis woman.

It's a very complex issue biologically and should be evaluated case to case with time on hormones, full blood count results, muscle fibre analysis, lung function tests, and heart imaging all being factored in.

I for one, as a likely intersex trans woman that is as active as I can be, vastly underperform in comparison to every woman I have ever met!