<p>I won’t say it completely doesn’t matter, because every program and every professor has their own idiosyncrasies. It’s possible that it might, just like it’s possible that it might make a difference when you are applying to programs if you are 50. It’s not something that any program will come out and say, but some professors may worry that a 50-year-old will not finish the program because of family obligations or just being “too old,” and even that if they do, that they won’t be able to get an academic position afterwards. Likewise, there may be some professor on some committee out there that will wonder if an 18-year-old will leave after a year or two if they get bored, or if they will be easily distracted in the program, or if they won’t fit into the department.</p>
<p>I will say that I’ve never heard of it mattering across the board. Their questioning your maturity and readiness for graduate school can easily be set aside by doing the same things as your 22+ competitors - being involved in research, forming relationships with professors and getting good recommendations that can attest to your maturity level and readiness for graduate school, writing an independent thesis, etc. It may also be good for you to take 1-2 years off and work as a lab tech doing research; you would still be younger than most applications at 20-21 but you would still have more preparation than most out of graduate school. And you would still finish your degree at ~25-27 assuming you take about 5-6 years to do it.</p>
<p>I have to ask, though - are you projecting that you will finish university at 18 because you are currently 14 and preparing to entering university, or are you projecting that you will finish because you’re like maybe almost 17, a senior, and think that you’ll finish university in 2 years or less? If it’s the latter, I recommend that you slow down and think about graduate school less, and focus more on the task ahead of you (that is, finishing college). A lot of people think they will finish in 2 years and don’t. The other disadvantage is that you’ll get less chances to do research and develop relationships with professors if you rush through the degree.</p>