이모 does not necessarily have a 'younger feel' than 아줌마. '이모' means 'mother's sister' so some people call their mother's close friends '이모' as well even if they are not young. 아줌마 may sound offensive and it also depends on the age of the speaker. If children call a middle aged woman 아줌마, that sounds fine. However, if an adult calls a middle aged woman '아줌마', it may sound offensive. So I'd like to recommend you to use '아주머니' instead of '아줌마'. I've heard some people even call middle aged female server '이모님 at a restaurant. And it is not rare. I find it is more likely used by outgoing people. And I've never called 이모님 to refer a middle aged woman that I didn't know. Some of my best friends' children call me 이모. I think it is safe to say '여기요.' or '저기요' to call a server instead of '아줌마' and '아주머니' because I've met some people didn't like being called '아줌마' or '아주머니' even if they were middle aged when they were not married.

I’m writing this as I like your question. Well, I haven’t thought about this issue before but I totally agree with @Soohyeon Kim. In my case, I don’t really use “ 이모 “ or “ 아줌마”. As I know, it is because Korean women don’t like to be called as “ 아줌마 “ and I’m not really outgoing and social to call someone who I don’t know, “ 이모” Besides, 이모 can also sound a bit old for me, so it don’t really use both, based on the fact that Korean values looking or being treated as younger. As a result, I just thought I say “ 저기요 “ most of time. In conclusion, I guess it could be difficult to catch Korean’s age as a foreigner, so I would suggest you to say “저기요 or 여기요 “ unless you master Korean culture lol. It can be scary to say it. FYI, Vice versa, 아저씨 or 삼촌 for men can be offensive a bit :)