I think both sides have merit in their arguments, but I'd side with Sanz in saying that some players, i.e. Tendulkar/Warne/etc, are so good that those around them can see they are already upto standard.
These selectors aren't looking at their stats but their game, their technique, their intelligence...what have you. You gotta understand that cricketers judge other cricketers by certain aspects that may not be apparent to fans like us.
Furthermore, I believe if someone who is touted as awesome comes and fails, then that person would not have been successful really anyway. Maybe more successful than they otherwise turned out to be, but not what they were hoped to be. What Tendulkar became is something that will occur for any player like him because those players are special and will not whittle away in the test arena. These players with strong mentality and desire (as well as their talent), which is what is needed at Test level, will succeed whether they are 19 or 24. The difference being when the 19 year old is 24, he will be already a senior member of the side with a decade of good performances in front of him, whilst less time will be available once that 24 year old, (who may take less time to become a senior member but will have much less time than the 19 year old in test cricket.
Anyway, I think too much is built on the notion that failing is bad. No, failure is what you need to get better. Improving yourself is needed. Some players are special enough to handle the capacity of failing in Tests, others can't and will need a period in FC cricket to adjust. However, the latter will usually, in my estimation, not be that special player the selectors may be looking for and the former will be. Or at least tried in order to see if it will bear fruit. Of course, this is in the situation where India was and where they needed someone like Tendulkar to come trumps. Not Australia that they are so endowed with talent that they are obliged to start a lot of their cricketers later.
What you are essentially risking is that a youth may turn out like Tendulkar or that he may fail and then turn into an average player. Or that the player you thought needed time in domestic cricket will enter as an above average player and will stay there or abouts.
Dunno if Ive been clear, I'm half asleep.