Why to early teen boys say hi to me when I am not even paying attention to them?

I wear plain clothes and look decent, I am in my early 20s (F). When I walk on the streets or in the train often 13 year old boys try to come in front of me or make themselves visible to me, as I ignore them, they are 13 what do I care and one of them of a group of 13 year old boys will say “Hi!” And put their hand in my face if I am sitting. Why do 13 year old boys do this, I just ignore them and don’t know them?

Why to early teen boys say hi to me when I am not even paying attention to them?
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