What SHR970 said.....ditto.
If you have a reasonable expectation of growing, MAYBE the regular model will be best. But - It is a lot easier to add a butt pad to a short stock than to do a good job cutting a long stock down shorter. I've seen professionals do that job poorly(and be strangely proud of their poor work).
For my own part, a 12.5 inch length of pull is ideal, 13 inch is manageable. It is a lot less comfortable to use something longer. 13-3/4 is great for full sized adult males. Not so good for their wives, children, or shorter friends. Or me.
Not familiar with the current full line of 870's specifically, and like many purveyors, they don't list LOP in the descriptions. Been a while but like most shotguns, the normal LOP on an 870 is fairly long. [Pet peeve - LOP should be a stat in every spec listing, at least on the web pages of the individual models. Despite rumor they ain't standardized too well.]
When expanding this LOP search to other long arms, be aware that some of the dimensional changes include shorter barrels. One rifle I would like will probably require cutting the stock on the regular stock model, as the youth barrel is four inches shorter.
One of the adjustable tactical stock is a really practical solution that works well in accommodating the LOP issue for everyone. They are not available for many long guns.....but the real problem there is a nice walnut stock is just a lot nicer to look at.
And with all that said, Dad's old Sears pump has a 14 inch LOP.
It sat up for 20 years. Glad I never had it cut, cuz its been just right for my oldest.
He be taller.