Is it OK to serve Milkshakes, Smoothies, or the like to Bochurim before Davening Shacharis in general? Is it the same if it’s as an incentive for Bochurim to add in learning?
Before Shacharis, one may not eat at all. If not eating disturbs one’s kavanah in davening, one may be lenient to eat before Shacharis. Chassidim would be particular to eat before Shacharis in such cases. Certainly, in such a situation it would be OK to drink.
So in the case you mention in which there are smoothies for those who come and learn early and need extra strength in order to daven it would be OK. However, if it is just because they come to learn and not to help them daven (rather it’s just an incentive) it should be given out after davening.
בנוגע לשתי’ לפני התפלה, ראה דע״ת פט, ג. ערוה״ש שם כג. ועוד. וראה גם קונטרס צדיק אוכל לשובע נפשו עמ’ 18.