Greetings Deepak!

Yes, you can appear again to continue your 12th privately but I do want to tell you that if it isn't for stream change, it isn't worth it. For example :- if you did your 12th from arts side first and now you want to do it from science side then it okay. But if you want to do it again just to improve your marks than according to me it's not necessary because it will waste your time and energy. You should instead focus on higher education. rest is up to you, you can do your 12th again from private no issues in that.

Hope it helps!

Yes you can dear aspirant, you can apply for 12th at any age. But do follow guidelines for 12 examination provided by your respective board of education.

After 12 you can apply for any bachelors course respective of your stream.

GOOD LUCK!!!