Welcome to the wonderful world of IRC (InternetRelayChat).
mIRC is the client (software) you use to get to any of many Networks by way of their servers. Each network can only be accessed by their servers. Once on that network, you can find a variety of channels and people, some talking about specific subjects, from the same age or general area, or just general chitchat. Finding one with people you are comfortable with isnt the easiest task, so be patient. It isnt unlike going to a huge party IRL (In Real Life), some groups of ppl (channels)you'll enjoy, others you wont.
One way to narrow your search is to get lists of channels with a specific word in their name or topic, like /list *texas* or /list *engineer* or /list *60+* (the * are called wildcards, so if a channel has the word engineer or engineering there, they would be listed). Join a channel, read the topic, and sit back to watch for a while, get an idea of whats going on and what kind of ppl are there. If you dont like it, try another channel.
You ask if you will get "teenage drivel", yep. You will also get ppl old enough to act like they have good sense but dont. You will find ppl who seem to think IRC is a kinky free-for-all. Imagine in place of "what's your sign" they use "asl" as their clever opening line. (asl== whats your age, sex, and location). Like IRL, you will also find some wonderful ppl who will add amazingly to your life. It gives you the opportunity to meet and get to know ppl from all walks of life, all ages, and all over the world. One of the nicest things is that you can talk to ppl and get to know them without preconcieved notions based on their age/ appearance/country of origin. You will find yourself engaged in great conversations, never guessing the other person is half your age or twice your age.
Give it some time, wander about different networks and channels.