Now this can be a problem for a man at any age, I don't care if you're 18 or a 28 year old man you could still be in the process of trying to squeeze out your first full beard. Discarding the actually physiological imbalance for a second (I'll get into those later), let's start with start with the most obvious reasons why your mom has more facial hair than you and work our way into the more dark arts of beard growing.
Keep It Clean
I know this seems fairly obvious but you would be surprised at how many people just disregard this as a real hindrance in their beard growing journey. I think it's because they focus on the wrong issue, they are probably referring to their skin getting to dry and that causing an issue but actually the bigger problem comes in when you skin gets too oily. As the oil builds up it begins to start clogging your pores and thus interfering with the hair follicle itself. I'll use this analogy even though I've ran through it a few times in my heard and there is no way to make this not sound crazy. Imagine if your parents dumped you in a box and through a bunch of mud on you, how easy would it be for you to grow... Good look forgetting that weird mental picture.
Check Your Testosterone
It's never too early to get your hormone levels checked, you would be surprised at how many people have hormonal imbalances that can inhibit hair growth among other things. Whether you grew up with a crazy home environment (like the kid in the box above) or you were like me and you experimented with all sorts of over the counter pro hormones that the guy at the supplement shop said are "safe" then you've probably done more damage then you realize.
As a man if you at any point mess with your testosterone level then you can pretty much guarantee that it's going to be permanently out of wack for the rest of your life. It's not the end of the world though, just schedule an appointment with your doctor and have them do a full hormone panel, if the test comes back with Low T or elevated estrogen levels then a form of TRT (testosterone replacement therapy) could be an option.
Hair Growth Serum
I know what you're thinking, this serum stuff sounds like an amazing new thing that you have to buy, in reality I just couldn't think of another word to use. It's actually just Rogaine foam that you use on your beard. There isn't a ton of research backing it and it's not FDA approved so it's one of those lovely little gray area's in life where you get to turn your body into the experiment. Do you research first, you can check out the post I did on it by clicking rogaine foam above because there are some periods that will scare you, like the shedding period. Don't worry, it will get worse before it gets better than life will be good again.
If you have these three areas in check then you shouldn't have any issues growing a huge beard faster than you were before.