Should I accept the imamate of the mosque despite my feeling of inadequacy and fear of falling short or being replaced?

If you are capable of assuming the responsibility of leading the prayer (Imamate) and the mosque community approves of you, then it is permissible for you to do so. It might even become obligatory if there is no one else. You must not say anything except what you know to be true. If you are not capable, then do not undertake it, especially if there is someone else who is qualified for it. The matter pertains to competence, not age. You may decline it if someone else is available.