So let's go to the point. Every time I meet and talk to older person (like some senpais) that we meet nearly everyday why do I always feels so reserved and it's hard to communicate with them?
But I could easily talk about many things with a friend on the same age as me (or maybe just a year age difference) that I just meet a day before.
If you know the solutions please help me, thank you.

maybe you're just afraid they might think of you as uncivilized person? or maybe you just want them to know that you're unlike them who are the total opposite of what you do? they are just maybes btw

I used to be just like you until I started working with my mom outside of school. It's just because you don't have enough interaction with them on a regular basis. There aren't any obvious solutions that I can give you other than to look for a retail job or to do some volunteer work around older kids and adults. You'll become more accustomed to talking with them over time so long as you keep an open mind.