Lol, I personally do not buy what Twilight said " this spell is only for highest level Unicorns ", come on, she is the most talented and powerful Unicorn in Equestria, we did not got any proves that there are any stronger ones even if ore experienced, if Twilight could not handle age changing spell, unless Unicorn special talent is time magic, there is no way any pony except princesses can perform this spell.
And if age changing spell is high level like 75 level spell, then Twilight must be on level 72, with level 100 as MAX, Alicorn Amulet just hax Trixie level to 100, without it she is hardly level 32.
In the Flash back episode that showed how they got their cutie marks Twilight overloaded herself and did an age spell along with other spells we never see again until the third episode. Twilight probably is a high level unicorn in all honesty she just hasn't fully grasped her power.
To be, it looked like Twilight was nerfed in that episode, I mean, in first episode where Twilight show her power, she lift 50 tons Ursa Minor and levitated it to everfree forest, but in this episode levitating many animals at once cause her to sweat, not to mention that she sweat and get tired really fast in that magic duel, I just fell that as much as it was really good episode, they made Twilight far weaker then she should be, that is why line " age spell is only for highest level Unicorns " bugs me, there is no way after so many years of training under Celestia, Twilight would be unable to perform spell what she performed when she went overload as filly.
It's not too unrealistic. A similar thing happens to people. Have you ever done something cool compleatly on accident and when you try to do it again you can't ? It safe to assume that an age spell is difficult so even if she was a level high enough it would probably take some time to figure out how to preform the spell.
She is not that great, I bet there are many experienced and strong Unicorns, just none of them have as much raw power as Twilight, after all they magic focus on special talent, they can not be too good out of they speciality, Twilight on other side is not a specialist, she learn about everything possible.