TLDR at the bottom

I’m 18, a cashier, and I’ve been working at the same privately owner pharmacy for 4 years. my mother is the front end manager of the store I work in.
I haven’t had many problems with my job, other than this coworker. We will call her L.

So my boss &amp; manager hired L and things started off great. She came in on time, was enthusiastic, asked us how she can improve, etc. after a few weeks this all changed. She would talk as if she knew more than the pharmacists and that got her reprimanded by our boss.
 Later on she started coming in late, bc she would stop by McDonald’s to get breakfast. Everyday. She’s heavily overweight (about 5’5 and over 350lbs) &amp; diabetic. She will stand at the front counters &amp; eat sugary/fatty snacks all day and drinks about 3 Mountain Dew’s &amp; 2 Coca-Cola’s a day. I mean to each their own, it just bugs me for her to stand there and eat instead of helping customers, answering phone calls, doing work, etc. We told her whatever time is on the schedule for her, she needs to be in here BY that time not AROUND that time. That didn’t help. If anything L started showing up later.
 I have an eating disorder, depression &amp; anxiety + insomnia which I’m currently getting help for. L tries to act like she knows me better than me, knows what my diet should be,  and what meds I should be on. I’ve told her off multiple times over the past 7 months for it.
L has recently gotten super childish. she’s single, has no kids, lives with her 2 sisters, her younger sister’s husband, and L’s ex boyfriend, so I get why she would get a bit antsy. But she’s 54. And she acts like a child. She will scare us if we go down the hall, flick rubber bands at us when we’re working near the front shelves, etc. 
today when I was helping a customer, talking to them about insoles they were buying from us, and I stretched my leg backwards slightly and L grabbed a wooden ruler and slapped it against the top of my thigh. Right where I cut(over recently made cuts) I know she doesn’t know I cut but even if I didn’t it’s still dumb to do that to a coworker trying to help a customer in a professional environment. So as soon as the customer left I pulled L aside and told her as calmly as I could to never ever do that again because it’s beyond childish to do that, and that I wasn’t being payed to babysit her (my side job is baby/house/pet sitting that’s why I say that). She got upset and started pouting. A 54 year old adult pouting because of an 18 year old. What the fuck?? No apology. She just walked away and sulked until she left. 

AITA for telling her off for being childish?

(Will be posting a separate story after regarding the same coworker)