Your type might evolve, and thats fine, but it rarely happens (especially among women). I have seen guys grow into leading men, but again, it is rare.
The expectations of how teenagers handle their types, is obviously different, but I think most training programs want to see a teenage leading man who is working towards his type - than have him come in at 6’ with broad shoulders and a beautiful face singing some Goofy character tune.
That makes sense.
@TheaterHiringCo - I think personality may count for more than 10% in determining type. Look at William Daniels - hardly an imposing physical presence in the classic leading man sense, but fully capable of dominating the stage through force of personality as John Adams in 1776.
But too be fair- John Adams WASN’T leading man-ish. He was short (2nd shortest after James Madison I think) and chubby…
My point was that there are leading roles - lots of them - which are not based on classical physical types, more on personality, and the components that go into determining type may not be 90 percent physical, 10 percent personality. Willum Defoe plays a really menacing villain while standing about 5-8 (his program height may be listed at 5-10, but I have stood next to him in an airport security line and he looked barely 5-8).
Also, Brent Spiner played an almost 6 ft tall John Adams, so casting directors do not generally worry a whole lot about historical accuracy (other than for maybe George Washington and Abraham Lincoln).
Type is really important for college auditions. You want to find monologues that fit your age/sex/type. It is really fun to be a young caucasian woman playing Othello against type when you are in college or are a member of a really cool theatre collective, but when you are in the audition room for college they want to see YOU, they know you are not a polished actor at this point – they want to see your personality and physicality and then they will decide if they need your type for the upcoming class. This is why on all the college sites they will ask you not to use an accent and to pick a monologue that is believable and that you connect with. This being said, you can still find really fun, different monologues that you can pull off – always be on the look out for new plays/playwrights. My d did a monologue from a play that was having its world premiere in our mid sized town. It then moved to off broadway. Many of the auditors knew the playwright, but not the play. It was very well received and it fit my d’s type and she loved it (which made performing it hundreds of times bearable!.)
@EmsDad - you are entirely correct. I guess I would say that there are “leading roles” in shows that are note any for traditional “leading man/lady” types. Tracy in Hairspray being a classic example, though off the top of my head I can think of more if those for guys than girls. But what else is new 
I think it is hard for MT high-schoolers who tend to be superfans of the genre to narrow down their type. My boys loved to sing anything and everything but it took them a while to realize what pieces were really suited to them. Just because they “could” sing it, didn’t mean they “should” sing it. A specific example: One of my boys loved singing “I’m Alive” from N2N (Aaron Tveit), but was told by a trusted advisor that while he sang it beautifully, unless he hit the gym and put on some shoulders and arms, it might not be the best song for him at that time (as a lean 18-year old boy). (Interestingly, since then, he’s done just that…and might perhaps get different feedback today.) This is another reason why third-party feedback is helpful – even if you don’t use a coach, it is good to run your material with trusted teachers, professionals and advisors familiar with acting/MT who can give you honest feedback on your song and monologue choices.
Also, don’t obsess about trying to find something “different” and “unique”. As a HS student, that was my S’s first concern - he wanted to stand out, look like someone who had “done the research” - and he ended up nixing some “tried and trues” that would have been perfect for him. He now believes his acceptance “yield” may have been greater if he’d been truer to “type” and respected his “actual” vocal range - which was pretty limited at the time (but getting better all the time - have no fear all you "Johnny One-Notes out there). And yes, @MTTwinsinCA, the trips to the gym are making a difference, too. While many HS girls may already being developing into their “type”, those lanky boys whose voices changed late seem to come into their own a few years down the road.
Absolutely, positively YES. Play your type. If there is ONE thing that I can drill into a student’s head that wants to audition for a theatre program, is that they MUST know and play their type.
I think it’s also important to note that the OP is asking if it’s “helpful”. Not vital, not critical, not essential…just helpful to know your type…I think it is helpful because it will increase your confidence level when auditioning. If you don’t know your type, then figure out what ISN"T your type and start there.
Adding onto my post #27, unfortunately casting in HS and youth theatre doesn’t really help kids fine tune their “type” because they play against type all the time. Only in these programs would you have a 14-year old Jean Valjean in “Les Mis,” a 17-year old Papa in “Fiddler,” or a 16 year old Mama Rose in “Gypsy”! 
One suggestion is to look online at videos from college senior showcases, and from programs like the national HS Musical Theatre Awards (not the character songs, but the individual finalist songs) or the Young Arts awards to see what different kids are singing and to get ideas of songs and types – you don’t have to sing the same songs, but you can look up the composers or the shows to find others. Also look for videos of new musical theatre showcases – there are many – to find new works that might feature young-adult characters/types.
@TheMTProject - I’m not a moderator, but I’m sure someone who is will evaluate the link.
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