Before I even start to comment on the tropic at hand, I felt that I should address this first. On the whole most of what Saracen has said is what I would agree with and yes Rolex mass produce all the components/cases, but at the end of the day, they are hand assembled and tested.
In comparison to an AP, Patek, Glashutte Originale, even D.Dornblüth (the later two being of German make as are Lange), the Rolex movements are nowhere finished to such an extent. However, Rolex wristwatches have stood the test of time and are extremely accurate and durable, and for the price you are getting an in-house manufacture.
However fantastic Rolexes marketing may be I personally doubt they could be as successful as they are today if their watches never delivered. The movements are minimally finished, but you would be hard pressed to find a 'tougher' mechanical movement - with such a distinguished history to the likes of the common Submariner, GMTII (16710 and older), DateJust, and even the Explorer series.
The Sea-Dweller in particular used to be one of Rolexes most over engineered watches in conjunction, but they have as of last Thursday released the Sea-Dweller Deepsea which is now fully waterresistant to 3900m (in excess of 12,000ft). I just flew into London this morning from Baselworld 2008 and can tell you that the SDDS is a truly 'chunky' watch that looks great in person, but I'm still skeptical as to how comfortable it would be on the wrist. It is simply a 'statement' by Rolex.
Do not be misguided though. Rolex is NOT a top-tier haute horologie manufacturer.
Let me further go on to state that the basic philosophy of a Rolex is as follows:
-completely waterproof oyster casing
-rugged and accurate movement, to less than +1s/day - this is the kind of performance I am getting on both my DJ and GMTIIC.
-timeless/classic looks
-retains its value very very well. It should not be a deal breaker but a definite bonus in owning a Rolex. Other brands simply cannot compete in this area unless we are taking vintage Pateks but that's a whole different discussion.
-Easily sold, if need me. In many parts of the world, some even consider Rolexes to be as good as cash.
Further to this, unlike a lot of the other brands the RSC in UK will finish and service your watch brilliantly. I tend to frequent the London RSC on a regular basis and I can only say great things about the service they deliver.
Now let's look at probably what would be considered the 'corner-stone' of watchmaking, Patek Philippe. When you send your watch in for a service, you can just about expect it back in about a years time.
Also if you want to read about a Patek 'nightmare' story regarding their level of service and how they treat customers who spend anything less than say a Skymoon Tourbion, please go visit TimeZone.com - there are many sad stories there.
However, at the end of the day buying a Patek is not the same as buying a 'reliable' watch. That's completely besides the point. You are purchasing a work of art, and like all hand made pieces problems do tend to occur more frequently, although it is of poor taste to discriminate the level of service to each customer based on the 'price tag' of their investment. For this reason, it's probably safer to own a 'simple' Patek, for the very reason of it being simple (less things to go wrong...)
Which now brings me to answer your question, and rather than dispensing advice on what you should or should not do due to your age etc. I am 25 and do not believe that there is an 'age' limit to enjoying the finer things in life. As long as you're mature and responsible, there shouldn't be a problem. Thankfully though, others have expressed their opinions in this area already.
So my advice, with regards to the question posed, go visit a couple ADs. AD = Authorized Dealers for example Omega, Rolex etc. Fraser Hart in Brent Cross have a wonderful display and extremely helpful staff. You may also try other stores such as Ernst and Jones, Goldsmiths etc.
Try on as many as you like until you find the one that sings to you. Easier said as done though as you have a budget to meet as well, so for your money I would focus on Omega.
A lot of the new models feature their co-axial free sprung movement, and this is even starting to move into the newer Speedmaster line. I personally stayed away from the 33XX movements due to various issues but I believe that these have now been sorted out.
A full service at Rolex costs £255.- and I have spent ~£200 + shipping to send watches to Omega in Biel/Bienne in the past.
You may also want to consider the added cost of insuring your watch and if you do decide to have it added to your parents home cover, made sure that it is added clearly to your policy and that you submit a jewelers valuation, which you can easily request for on purchase.
If you decide to go second hand, be very very weary as China makes more fakes per year than probably all of the Swiss watches to hit the market. Rolex produces approx. 1m watches per year, China makes 5m!
All the best,
Mike