Argh, this! I don't lie but people think I am lying!
I'm a doctor in a hospital. I look really young- my mum is the same, 60 and doesn't look a day over 40. I love this in real life, but at work it's a nightmare. I am 34 and have been training to be and working as a doctor in various guises since I was 17, with about 18mo off for ML total. I always introduce myself with full name and correct job title.
"Are you sure you're the consultant?"
"How long have you been here? I bet you're new, eh"
"Are you the work experience student?"
"Are you REALLY qualified to do that?"
"When will I see someone more senior?"
Usually a gentle smile and "Oh I've been a consultant here for about four years but I've worked in [health board] for twelve. Actually I am the most senior person here" shushes them but it still gets me down. I dress extremely conservatively and am sure that I don't behave like a 20 year old!
If I get asked my age directly I usually answer with "Old enough to have two children in primary school" which makes them start doing maths and get distracted.
Worst is when I'm with a male junior doctor. Patients will often ignore me and speak to them; which often makes them feel very uncomfortable.
On the plus side, when I'm in a hoodie and jeans I can still get a half on the bus 