Speaking from a theatrical background, stage-work, I will honestly say it's a combination.
The actor/actress being made up affects this. Do they have smooth skin or wrinkled, do they have a tendency to look younger/older than they are to start with? I find it's easier to make a younger person look older because it's a case of addition over reduction. To make someone look older, you can create laugh-lines, create drooping jowls or extra chin flab. You can add the wrinkles, play with the hair (roughing it and then whitening it works...or if you have liberties, even thinning it throughout the head, or balding them). When you're working in reverse, to de-age a person, you have to handle the reduction of lines and extras...
Another factor to account for is the difference of ages. It's very easy to age or reduce a person 5 to even 10 years, but a 20 year difference going either way, you're going to need serious expertise.
It's similar to drawing. In actuality, it's easier to draw or paint an adult than an infant, simply because of the features that age bring to our faces over the smoothness of infancy.
I had one fun experience when doing makeup on the "Noah" character for the show "Two By Two." Before the show, I needed to age the 40 something year old man to look like he was 900 years old. In a VERY short time, I had to de-age him to look 90 (well. 90 their time, more like 50 our time. So about a 10 years difference). Later in the show, I had to re-age him to about 900.
Effectively, I had to age him in all cases, but it was an interesting challenge...and as I'm not a professional, I found it to be a fun one...but it wasn't too difficult.
I have, however, encountered difficult situations. A production of "Drood" where Jasper was played by a 19 year old and Drood by a 32 year old. Jasper is supposed to be 6 or so years older than Drood. We opted to age the 19 year old and did nothing to Drood.