Newly returned swimmer - USS vs. USMS

Hi all! Apologies in advance for this likely-stupid question from a newbie. I'm a 21 year old, post-college swimmer who wishes to return to competitive swimming after a years-long hiatus. I did not swim competitively in high school or college. Thus, my only experience is with USS age-group swimming. I've spent the past two months training on my own (6-7 days per week/2 hours at a time). Although I have a demanding job, I want to push myself to swim at the highest level I can; my goals are purely competitive, not recreation or fitness oriented. I want to join a team of like-minded people. So, my first question is basic: USS or USMS? I've heard that USS is 18 and under; USMS is 19+ - USS also seems to administer the national competitions. Does this mean that the sole option for a swimmer 19 or over is to join a USMS team, compete in Masters meets only, and (if/when) they make national cuts, switch over to USS somehow? Or, do the USS teams also take swimmers over 18? If so, what would be the pros and cons to joining a masters team versus a USS team? Also - I'm assuming this is true, but are USMS times automatically valid for USS purposes and vice versa? Any help is appreciated. I'm sorry for how obvious the answers to these questions might be for someone who's been swimming for a while, but I've been trying to bend the powers of Google to my will for two weeks and yet haven't found the answers yet. Thanks. RH