I would say, yes they age normally. It's part of the natural order that things die after a certain time.
The size change at 4th or 7th level is not magical, it's more showing that the baby tiger now finally grew up into an adult one. At this point it crosses the threshhold from a big medium size animal into a small large size one.
Alignment: It's still an animal, and animals are "Always neutral" unless otherwise stated in their entry. Nowhere does it suggest that ACs have a different alignment, so they'd be N.
You're right it should be able to wear an amulet with Holy and Unholy at the same time. I'd be tempted to veto it as a GM though, but as a druid it makes some sense even.
Greetings, fellow traveller.
From what I gleaned from the rules, the AC continues to age as normal, thus would need replacement every now and then.
Since standard ACs lack intelligence to make willful decisions or develop any kind of moral standards, I'd rule alignment is based on the beastiary entry, thus it would be N.
Ruyan.
you don't have to have the same animal companion your entire career, you can change them out once a week if you wanted to
Animals act by instinct. Thus, they are True Neutral alignment. Now, once you give it a few points of intellect, well, it can develop an ethical system if it wants.
PS - I've also seen the idea that animals and similar creatures are given an 'unaligned' or 'instinctual' alignment and are completely agnostic to any of the bane/holy/Circle of Protection/etc abilities. They can't use them or be affected by them.
I think the flavor difference is that for a non-sentient weapon, alignment is basically a "taint" of extraplanar power - much like how good/evil spells work. It's more a measure of the power types involved than an actual moral question (look at the bazillion of threads on alignment and spells). For living creatures, alignment is heavily based on actions and on a scale from good to evil. For other things such as spells or items, it might be less of a scale and more of a checklist.
I'm not sure if a weapon can have several alignments and I agree that it seems a bit weird to have them, but it can be explained away flavorwise.
You misunderstood me.
If it's just a rule question, yes, you can have both holy and unholy on the same weapons. There's no rule that prohibits a weapon from being both good-aligned and evil-aligned at the same time.
The way you asked the question ("explain to me how a weapon can have 2 alignments?") seemed to me to be a question of "how does that make sense?", that's why my answer was the way it was. And I didn't mean it as a "moral" question, I meant that when it comes to sentient creatures, alignment is a function of morald. When it comes to, for example, spells, they are not in the same related - this could be extrapolated to also include items.
But for the answer on RAW - yes, a weapon can be both good-aligned and evil-aligned at the same time. I don't know how to answer the question "how" more than "for example, by having both holy and unholy enchantments". Why ask how when you yourself explain how?
Note that it does not say "the weapons alignment becomes good", but rather that it is good-aligned - while these may seem the same thing, in other cases where sentient creatures are referred to, they explicitly state "alignment becomes X" rather than "x-aligned".
And this isn't really corner-case; having a "weapon of balance" that kills both good and evil is almost a trope by now, being featured in at least a halfdozen games or so.
Oh lookie lookie herie herie
EDIT: And more on alignment on creatures vs nonsentients here