First of all, I am sorry I could not find a complete reference for this fact: I think with some effort one can find something similar in Petersen, Riemannian geometry, Lee, Riemannian geometry, an introduction to curvature or Gallot, Hulin Lafontaine, Riemannian Geometry. But I could recover the result that follows ; there may be some typo. This is quite long but really is elementary.
Suppose $J$ is a Jacobi field along a geodesic $\gamma : I \to (M,g)$. We fixe some notations: we denote by $\sec$ the sectionnal curvature of $(M,g)$ and by $R_{\gamma}$ the tensor along $\gamma$ defined by $R_{\gamma}X = R(\gamma',X)\gamma'$. It is equivalent to say $g\left(R_{\gamma}X,X \right) = {\|X\|_g}^2\sec(\gamma',X)$. Recall the Jacobi equation is
$$
J'' +R_\gamma J = 0
$$
The function $t \in I \mapsto \|J(t)\|^2$ is smooth, and we have
\begin{align}
\dfrac{\mathrm{d}^2}{\mathrm{d}t^2}\left\|J\right\|^2 &= \dfrac{\mathrm{d}}{\mathrm{d}t} 2g\left(J',J\right)\\
&= 2g\left(J'',J \right) + 2g\left(J',J'\right) \\
&=-2g\left(R_\gamma J, J\right) + 2\|J'\|^2
\end{align}
Moreover, the left hand side can be computed another way for $t$ such that $J(t) \neq 0$:
\begin{align}
\dfrac{\mathrm{d}^2}{\mathrm{d}t^2}\left\|J\right\|^2 &= \dfrac{\mathrm{d}}{\mathrm{d}t} 2\|J\|\|J\|' \\
&= 2\left( {\|J\|'}^2 + \|J\|\|J\|'' \right)
\end{align}
and this tells us that for $t$ such that $J(t) \neq 0$, then
$$
2\left( {\|J\|'}^2 + \|J\|\|J\|'' \right) = -2g\left(R_\gamma J, J\right) + 2\|J'\|^2
$$
from which we deduce
$$
\left\|J \right\|'' = -\frac{g\left(R_{\gamma}J,J \right)}{\|J\|} = -\sec(\gamma',J)\|J\|
$$
Suppose $M$ has positive sectionnal curvature bounded from below along $\gamma$, say $\sec \geqslant \kappa^2 >0$, and define on $I$ $f(t) = \|J(0)\|\cos(\kappa t) + \|J(0)\|'\frac{\sin(\kappa t)}{\kappa}$. It is solution to the second order ODE $y'' = -\kappa^2 y$.
Now appears the trick: consider $g(t) = f(t) \|J(t)\|' - f'(t)\|J(t)\|$. Then on an interval containing $0$ on which $f(t) \geqslant 0$ and $J(t) \neq 0$ we have
\begin{align}
g' &= f\left(\|J\|'' + \kappa^2 \|J\| \right) \\
&= f \left(\|J\|'' + \sec(\gamma',J)\|J\| - \sec(\gamma',J)\|J\| +\kappa^2\|J\| \right) \\
&= f \left(\kappa^2 - \sec(\gamma',J) \right)\|J\| \geqslant 0
\end{align}
Hence, $g$ is non-decreasing and $g(0) = 0$. This shows that
$$
\forall t \text{ as above},~ g(t) \geqslant g(0) = 0
$$
which turns out to be
$$
\forall t \text{ as above},~ \frac{\|J\|'}{\|J\|} \leqslant \frac{f'}{f}
$$
integrating gives
$$
\ln \frac{\|J\|}{\|J(0)\|} \leqslant \ln \frac{f}{f(0)}
$$
and as $f(0) = \|J(0)\|$, we can deduce that for all $t$ such that all the above works
$$
0 \leqslant \|J(t)\| \leqslant f(t)
$$
Now, you can deduce vanishing properties of $J$ thanks to this inequality.
Also, the exact same study in case the sectionnal curvature is bounded from above $\sec \leqslant -\kappa^2 <0$ shows that
$$
\forall t,~ \|J(t)\| \geqslant \|J(0)\|\cosh (\kappa t) + \|J(0)\|' \frac{\sinh(\kappa t)}{\kappa}
$$
Comment: if $J(0) = 0$ one can adapt this proof to fix this. Also, $\|J\|$ may not be differentiable at $0$ if $J(0)=0$, but if the curvature is of constant sign, then above calculations imply that $\|J\|$ is concave or convex, thus differentiable from the right at zero and we are done.
Another comment: if $\kappa \geqslant 0$ without having a lower bound, we cannot say much, because the euclidean case shows that $J$ may not vanish for positive $t$.