My take is that most supplements are not going to lead to any noticeable outcome unless there is an insufficiency of a specific compound for whatever reason, insufficiency which is usually unlikely unless malnourishment or a pathology.
Another factor is health, for example if someone testosterone is very low there are quite a lot of supplements able to return it to normal levels, something as simple and inexpensive as onions do work wonders in this context.
But if testosterone is in normal range in order to rise it even a little bit further I can only wish you good luck with supplements...
If someone intake level of vitamin C is already adequate by means of correct diet supplementing ascorbic acid is very unlikely to produce any noticeable positive effect.
Maybe about some supplements the fact that they don't show any NOTICEABLE effect doesn't necessarily mean they don't induce positive influences on some health factors which in the long run do make a difference indeed.
Another thing about skin rejuvenation I noticed, there are a lot, really A LOT, of compounds that do show in studies very noticeable improvements in elasticity, thickness, inflammation and so on...but is very unlikely the effects are cumulative.
It means that if compound y improves skin parameters by 15% and compound x improves skin parameters 18% it doesn't mean that using both together or one after the other one in cycles leads to a 33% improvement, likely not even to a 20% since very likely they effects the same causes even if sometimes by way of different routes.
In this perspective I think it is relatively easy to find a compound which in studies shows beneficial outcomes on subjects whom are in a relatively "bad shape" and never did much about it, a whole another story is to achieve improvements on someone whom is in "good shape" for its age and/or is already taking care by mean of supplementing or by the use of beneficial topical compounds.
It is a bit like in training, it is easy to double in a relatively short time the strength of someone totally untrained and unfit, a different story is to improve the strength even only of 10% in an experienced powerlifter.
My take home message is that a study showing promising outcomes doesn't necessarily guarantee a particular compound to elicit any improvements to everybody, in many instances is just adding more water to an already full bucket: it is not going to make the bucket any fuller.
Rosmarinic acid likely works...like many other compounds do...for someone "new" to supplementing likely will show interesting outcomes but it is nothing really "special" or "unique"...same or similar results are likely achievable by other means and adding it will not make any dramatic change.