Tell them that as their not in the business, they have no legitimate reason to make those kind of claims. Ageism might be an issue in plenty of fields, but it’s not in the web industry. You could be 8 years old or 80, what matters is that you have the skills to do your job. Our industry is based purely on experience and knowledge, age is not a factor. Just because it’s a popular career choice for the young does not mean that all the old people need to disappear. Just look at Jakob Nielsen I doubt he would mind anyone pointing out he’s 52 and it feels like he’s been around since the dinosaurs but he’s one of the worlds foremost leading experts on web usability. 
I don’t agree it’s an easy profession to start working in. While there’s not any requirements to practice or certifications taken seriously, we do have a professional body of sorts any coder worth their salt adheres to (the W3C). Yes there’s a lot of wannabe’s and a lot of misconceptions, but I would argue that there’s no industry or career in existence which has such a fast turnaround for knowledge and such a competitive edge to follow suit. More and more people recognise good and bad design and are becoming more web aware, and if you don’t know your stuff or think you’ve learnt everything, your going to find yourself out of job opportunities soon.