Summary:[One day, a companion asked Imam ar-Reza (A.S.), “If something happens (to you), whom shall we refer?” Imam (A.S.) said, “To my son Abu Ja’ffar”. Imam al-Jawaad (A.S.) was still a child.
In the matters of Prophethood and Imamat – age carries no value and this was made clear by Imam Reza (A.S.) during his lifetime.
روي عن أحمد بن محمد بن عيسى عن أحمد بن محمد بن أبي نصر قال: دخلت و صفوان بن يحيى على الرضا عليه السّلام؛ و أبو جعفر عنده نائم له ثلاث سنين فقلنا له: جعلنا اللّه فداك انّا- و نعوذ باللّه من حدث يحدث- لا ندري من القائم بعدك؟قال: ابني هذا.فقلت: و هو في هذا السن؟
فقال: ان اللّه تبارك و تعالى احتجّ بعيسى ابن مريم عليه السّلام و هو ابن السنتين و ان الامامة تجري مجرى النبوّة
[One day, a companion asked Imam ar-Reza (A.S.), “If something happens (to you), whom shall we refer?” Imam (A.S.) said, “To my son Abu Ja’ffar”. Imam al-Jawaad (A.S.) was still a child. So the man said, “I think he is still very young.
Imam ar-Reza (A.S.) replied, “Allah sent Jesus, the son of Mary as a prophet though he was younger than Abu Ja’ffar will be when he becomes the Imam”.] (Hayaat al-Imam Muhammad al-Jawad)
What the Imam (A.S.) implied was that Allah (SWT) chose Jesus as a prophet and gave him knowledge while he was just a child and younger in age than Imam al-Jawaad (A.S.). Prophethood and Imamat are from one and the same source. Allah (SWT) does not see who is young or old, rather He (SWT) entrusts him with this responsibility whom He (SWT) likes!
Footnote:Esbat al-vasyet, Masoudi, p, 220
Source:https://groups.google.com/forum/#!topic/shiagroup/7GlY4I-t5q8