There are no limits to voluntary lectures. It does not matter the age if the lecturer masters the topic and engages the audience. However, the question is sufficient knowledge on the topic, certain "speaking" skills. The curse of the lectures is a monotonous colorless voice without gestures, without pictorial attachments ... Simply a lecture to sleep. A young active lecturer, take the first steps at school before your year, for example, try a health lecture on the influence of puberty on the relationship between a girl and a boy. It is a topic relevant to your year and the listeners and listeners will certainly contribute with their experience. You want to be with seniors, it's actually similar to school, some people are interested in the topic and join the discussion, some don't even register that they are there, some they come up with their experiences , some of them may even scold you ... prepare for the lecture, it must not be long, study the topic, get to know "enemy and friend" and know how to distinguish them, study a certain pantomime-oratory speech, some seniors see worse, some hear worse and some both, but the word associated with movement is more interesting . Good luck, PS. don't look for money for it, certainly not at the beginning, maybe only after reaching the Nobel Prize.