I want to delete some of my old questions that are off-topic and received downvotes. Frankly, they were bad questions because I was not so mature 3 years ago.
Can someone please tell me what I should do? Should I flag them with deletion request?
From the FAQ on Meta SE on deletion:
You can't delete your own question if it:
- has an answer with upvotes (even if that answer has a net zero or negative score)
- has an accepted answer
- has multiple answers (even if there are no upvotes)
- has an answer with an awarded bounty
- has at least one other question that is marked as a duplicate of your question
You can't delete any of your own posts if you've already deleted five of your own posts on the same day.
If your post does not satisfy any of these exceptional criteria, then you can delete it by clicking on the "delete" button beneath the question, alongside the "share", "edit" and "flag" buttons.
Flagging a moderator for deletion is not the way to go, because if you can delete your own post then you should take the action yourself, and if you cannot then there are probably good reasons for why that is so. The FAQ on deletion has some more details on this point, you can see the section titled "If I flag my post with a request to delete it, what will happen?"
Even if you have the ability to delete your old questions that you feel are off-topic and/or are poorly received, consider trying to improve them using the knowledge you have gained so far. This will be a gain for both yourself as well as the community.
Indeed, as @amWhy points out in the comments below, one's deleted posts are also taken into account when one receives a post ban for low-quality contributions. Though in your specific case this doesn't seem to be an issue, most new users are unaware of this fact and think that deleting their downvoted/closed posts will help bypass the ban, whereas it is not so. Hence, the advice to improve one's poorly received posts is to be doubly encouraged.
The FAQ on post bans goes into greater detail regarding the above points. It might also be useful to go through the suggestions given in the Help Center article I've thought better of my question; can I delete it?.
If you are absolutely sure that you want to proceed with deletion and are unable to do so yourself, you might try posting links to those questions in the chatroom CURED and requesting help in closing and deleting. (I once did so for one of my questions that was an $n$-fold duplicate.)