I don't mind sponsoring adults doing things, and I've done a couple of events that have required me to ask for sponsorship so I know how awkward it is asking too.
I have a loose rule that I'll sponsor one person per month and generally I give about £10-£20 depending on the charity ( more if I support it and like it and less if I don't ) and the difficulty of the event. If its a close friend, niece/ nephew /sibling I'm probably more generous.
I often get more than one request a month ( marathon season etc) and I then don't feel guilty not contributing.... and I also sometimes explain to people I'm not sponsoring this for reason but I wish them well for their event.
I really like it when people show they are trying hard with sponsorship and make some effort themselves such as raffles, quizes, cake sales, curry nights, litter pick ups, car washing, vinted wardrobe clearouts, car boot sales etc. This makes it more likely I will support and sponsor them!
I have another rule,,, it someone I sponsor doesn't thank me by messenger, email or in person I will never sponsor them again. Rude! It happens alot!