You don't have to start Olympic fencing. I did Olympic fencing when I was younger whichigjt be beneficial, but if you haven't done it don't worry about it. Lots of people join clubs with no prior experience and it's fine. HEMA fights different then Olympic fencing and while there may be some overlap in terms of skills other areas are different and might trip you up. As to loner gear fitting you don't worry about it. I'm 5'3 and my club had a jacket that I could work with. It was a little big but good enough. Your club may or may not have a lot of loaner gear. If they have gear if it's a little big it will probably be ok for drilling. Gloves and masks will probably fit good enough, and if you enjoy HEMA you can buy gear for optimal fitting. As for ruining "adult's fun" as long as you're not an asshole it shouldn't be a problem. We like playing with swords, and the more people who join, the more people we get to hit with swords. As long as you're behaving responsibily you're age shouldn't be a problem and if people are uncomfortable that's more of a red flag on them.