The same question was asked on Expats sister-site, Travel Stack Exchange, and responded acknowledged expert @GayotFow (with permission):
I assume you are a non-visa national, in this case one of the countries participating in the YMS and want to apply for leave-to-enter at the airport. If they let you enter as a tourist (more about that later), you will become an inadvertent overstayer (that's a bad thing) when your leave-to-enter expires. Also, since the leave-to-enter stamp will be the only one in your passport, you will have difficulty establishing your start date as a YMS visa holder (also a bad thing). In order for your YMS to be valid, your entry clearance needs to have an IO's stamp on the same page and covering part of the entry clearance itself.
The work-around is to channel hop and re-enter on your YMS start date. They will not be happy about it because it will look like you are attempting to get more leave in the UK than you were issued (which is precisely what you propose to do). The outcome of that discussion when you re-enter is governed by personal impact and articulation skills and cannot be speculated upon here. If things go badly and if you are from one of the countries that requires a YMS sponsor, they like to call the sponsor and let them know what you're up to and give them the opportunity to revoke your certificate. Sponsors like those opportunities because they think it keeps them in good stead with the authorities. So if you do have a certificate, make sure your sponsor is aware of your plans.
Getting back to entering early, we turn to Paragraph 30C of the Rules..
An Immigration Officer may cancel an entry clearance which is capable
of having effect as leave to enter if the holder arrives in the United
Kingdom before the day on which the entry clearance becomes effective
or if the holder seeks to enter the United Kingdom for a purpose other
than the purpose specified in the entry clearance.
It's possible you could be vulnerable to both of those conditions: entering early and entering for tourism. But again, these things are down to the Immigration Officer's discretion. The rule uses 'may' which means the IO has the option, but not the obligation, to cancel your YMS. And if you get in, you will need to channel hop and face a different IO when you re-enter.
Adding: the question is topical [on TSE] because there's a tourism angle on it; generally however, YMS questions that have to do with working or living in the UK for a long time should be asked at Expats.