  In this paper, we investigate the possible theoretical constraint on the
parameter $n$ of the agegraphic quintessence model by considering the
requirement of the weak gravity conjecture that the variation of the
quintessence scalar field $\phi$ should be less than the Planck mass
$M_{\rm{p}}$. We obtain the theoretical upper bound $n\lesssim 2.5$ that is
inconsistent with the current observational constraint result $2.637<n<2.983$
(95.4% CL). The possible implications of the tension between observational and
theoretical constraint results are discussed.
