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Hello Marwa.Mohamed,
You can say '...since I was five' or '...since I was five years old' here. Neither is informal, though the second sounds a little more offical than the first in my view.
Your sentence is not incorrect but I think the present perfect continuous would be a more natural choice:
Best wishes,
Peter
The LearnEnglish Team
Hello Pavan Kaur,
Since the subject ('Task A and Task B') is plural, the verb should also be plural ('have'). Though I'd probably rephrase it slightly if I were writing it as 'Tasks A and B have to be done'.
All the best,
Kirk
The LearnEnglish Team