there are some artists where age is totally non-relavent, like the youngsters are closer to chibi than children, Inuboshi and Hoshino Fuuta for example. Those you'd really have to go out of your way to make them too young, because they wouldn't be younger in my perception, just smaller.
Example: Mikan, from Wanko to Kurasou. There's not a whole lot the artist could do to make her look younger.
Then there's stuff with more realistic artwork, where you can see actual muscular/skeletal detail, Naoshi Onizuka or Juan Gotoh for example. For that kind, I'm cool with early adolescent, or just about to hit puberty, but noticeably younger than that gets to be unappealing.
Image take from Lovable, by Naoshi Onizuka. This style of artwork could definitely show a child as being "younger" and not just "smaller".
And honestly, a lot of it has to do with the partner in question. The bigger the age discrepancy, the less tolerant I'll be of a younger loli.