There isn't an officially stated lifespan for Changelings (as of writing this answer).
The most important thing to remember is that Unearthed Arcana material is only effectively playtest material. It's not "officially" published or polished and thus isn't nearly fleshed out as much as the core rule books or supplements so you're not likely to find that level of detail until it's actually put into a supplement book, if it even does.
There's only one other mention of changelings that I can find and that relates to doppelganger children in the Monster Manual but even that doesn't give us any information on their lifespan.
However, if you really want something to base it off and get a rough idea then it has been detailed in the 4e Eberron Player's Guide (unfortunately I can't speak for older editions):
Changelings reach maturity at about fifteen years of age, and they live about as long as humans do.
The AC changes, to 13
In their human form, the PC weretiger with 16 DEX wears the studded leather armor, giving them 12 + 3 (= 15) AC.
They polymorph into their tiger form.
Any equipment it is wearing or carrying isn't transformed.
As you note, the armor does not transform to fit them, so they do not benefit from the human-shaped armor. It is the DM's discretion whether the armor is destroyed, damaged, or just hangs off the weretiger's new form uselessly after the transformation.
Since they are not benefiting from the armor any more, they can no longer use the AC calculation (12 + DEXMOD) provided by the armor. As Solias notes, in the Player Characters as Lycanthropes box on MM page 207, AC modifications in different forms are noted for other types of lycanthropes, but not for weretigers, so their AC is now simply the basic 10 + DEXMOD = 13.
Equipment is not a statistic
Your confusion seems to be centered around the following passage:
Its statistics, other than its size, are the same in each form.
And this is true.
If the weretiger were wearing a Ring of Fire Resistance which it took off and dropped exactly as it transformed, it would not still have resistance to fire in its tiger form, despite "resistances" being a statistic. This is because fire resistance is not a statistic of the PC, but something provided by the ring to the PC.
The same is true for the AC calculation (12 + DEXMOD) provided by the studded leather armor - it is not a statistic of the PC, but something provided by the armor to the PC.
Jeremy Crawford has tweeted to clarify that equipment is not an inherent part of monsters in the context of a similar question around the Shapechange spell - the same is true of PCs.
Best Answer
I would argue that the Changeling's Shapechanger ability is only skin-deep, in that it allows you to take the form or "shape" of the target humanoid, and nothing else. At best, it allows the Changeling to disguise herself as a different humanoid at will.
Taking into account that "you can polymorph" and "you can cast polymorph" are two very different things, we cannot equate the term "polymorph" as used in the Shapechanger ability with the Polymorph spell. As such, we define the term as we define most other terms that are not otherwise defined by the rules: as English words. And "polymorph (verb)" simply means "to take a different form".
To allow the Changeling to use the Polymorph spell at will is crazy and game-breaking. You've essentially achieved the Druid's capstone ability as a racial trait! ^_^