Let's now consider the most typical case of them: the calculator is for a student.
I hold the strong opinion that children should not use calculators until they master mental and written basica calculation: adding, subtracting, multiplying and dividing. That implies that they shouls have their first calculator around 12 years old. If they get to know that learning claculatin is not necessary, they will never do it. Your opinion may be different on this, but I think that there has to be a lower age limit.
After that limit, then there are two HP models that can be used: the HP10s+ and HP300s+. The difference between both is not substantial. Price is quite different, but none of them will break the bank. These can work until 14 years old.
Then, you need to do some investigation. Does his school support a particular calculator model? The school might support either a scientific calculator or a graphical calculator.
If the school supports scientific calculators, he will be extremely happy with the HP35s - probably the most beautiful calculator on sale now. It looks professional, and all his companions will admire him for it. It doesn't make any sense to endow him with a HP15c or 42s - he will not appreciate it, and more important, his companions will not appreciate it either. Peer opinion is extremely important at that age.
If the school supports graphical calculators, then you really don't have any options. The HP50g is more aimed to university technical studies, and is on its way out. And your student will never forgive you if you give him the brick that the HP39gII is in comparison with the sleek Prime. so, the Prime has to be!
No student can be disappointed with a colour, tactile screen machine like the Prime. And it can do games too - but he'll have to learn some machine internals to play them - which is good, too.