When numerically solving the Friedmann equations for varying cases of an open universe (i.e. $\Omega_0 < 1$) I get the following evolution plots,
where the left plot is for an open universe containing only matter and the right plot is for an open universe containing 30% matter and 70% dark energy. As expected, in the dark energy case, the scale factor exponentially increases with time (accelerated expansion of the universe).
However, when looking at the current age of the universe (the time at which the scale factor $a=1$) it can be seen that for each age in the left plot, the corresponding line in the right plot has a significantly greater age. I would have thought that the inclusion of dark energy would cause the age of the universe to be smaller than the corresponding matter only case due to the negative pressure anti-gravity properties of dark energy pushing everything apart.
If anyone can explain the reason for this, I'd appreciate it.
EDIT:
In both plots the blue lines correspond to $\Omega_o = 0.1$, however the left plot is $\Omega_{m,0}=\Omega_0=0.1$, whereas the right plot is $\Omega_{\Lambda,0}+\Omega_{m,0}=\Omega_0=0.1$ (with $\Omega_{\Lambda,0}=0.07$ and $\Omega_{m,0}=0.03$), where $\Lambda$ and m denoted dark energy and matter respectively.
The time axes aren't very clear, apologies, the left plot axis starts at negative because I've set t=0 as the current age of the universe ($t(a=1)$) to more easily compare the shapes of the plots - I have taken these plots from a paper I've written so they may seem strange out of context.