I always hate how people always jump to "but inflation..." or "12% is not realistic...". Ok, maybe 12% isn't reasonable and $1M won't be as much decades from now. But having $500k or $750k or however much you end up saving by saving a few bucks each day could still be a pretty good chunk of change when you are ready to retire! That's much more than $0! The earlier you start, let's say when you're 18 buying a $5 cup of coffee and not really thinking about retirement, gives you more time to build wealth when you're ready to retire.
As more people start to reach retirement age, there's going to be a lot of people surprised to realize they can't afford to retire. With few companies offering pensions and some uncertainty about social security with Baby Boomers reaching retirement age, millennials better start thinking about how they can retire as soon as possible. Saving a few bucks each day (whether it is by giving up fancy coffee or something else) can be a great way to get started!