The word old has many usages in English. English speakers often assume that you can just use Japanese words in disregard of differences of usage once they learn it means such and such. So, we get things like はちねんふるい人 by students who learn the words for "eight", "year", "old", and "person" and then put them together in a way they as English speakers are familiar with. This is despite the fact that the correct way to say this is 8歳の人, introducing a grammatical concept of counters that English lacks almost entirely. Ignoring other problems such as the fact there are other ways to say the previous words, let's focus on the word "old".
Consider the following phrases which all have something translated into English as old. Keep in mind that there are many more phrases out there that exist which equate in someway to old because the English word's usage is so broad. This leads us into understanding the importance of looking at how Japanese phrases things. We have to in some way detach ourselves from English to get the bigger picture.
Old politicians don't have endurance.
To care for an old dog and watch its final moments.
My father-in-law does not want to enter the nursing home.
Respecting the old/elderly is engraved in me.
To cut down old oak trees.
You become an adult when you turn twenty years old.
The night grew old.
How old is your child?
An/the old, wooden house completely burned.
To respect old traditions.
I came across my old boyfriend.
One problem English speakers have with Japanese is Japanese's lack of a simple pattern such as "# old" that can apply for anything. Depending on the object, the phrasing options are different.
Have you ever thought about the age of inanimate objects? There is nothing wrong in saying something like Ex. 12a, but Ex. 12b is also OK.
How old is the age of the Earth?
The answer, however, would always be 46億年. Using 歳・才 would be incorrect. If your dog is 11, he/she is 11歳, not 11年. This is because dogs are alive and like humans.
Your apartment complex may be a 5年物, not 5歳. You may also live in a 5年前に建てた家 or 5年前に建った家 depending on context. With this in mind, consider the following examples.
A hotel that was built 100 years ago
This is/it's a house built 300 years ago.
To live in an apartment of 50 years.
Phrase Note: 築＃年 specifically shows how long ago a piece of architecture was constructed.
Ten year wine
Word Note: The phrase 古いワイン exists as well.
This is a fifty year old wine.
Particle Note: The もの is 物. の can be dropped in this context. It is more traditional/correct with it.
A photograph form fifty years ago
Train cars from 30 years ago
Particle Note: The particle の may also be omitted here as well.
When you have something like 〇〇年齢は何歳, 〇〇の樹齢は何歳・何年, you get to use 歳 in the question. If you were to show a difference in age, 歳 is possible. Though, when the tree is far older than a human could ever be, ～歳 becomes impractical. Even so, 木の年齢差 is shown with ～年. ～歳違いの〇〇 or ～歳違っている, though, is common for animals and plants. It should be clear by now that everything is subject to pragmatic issues.
Applying ～歳 to something non-human personifies the object. We're fine with animals because we're animals. We're sometimes fine with plants because we have our tree lovers in Japan too. We're OK using it in a question when we specifically use the phrases 年齢・樹齢, but unless you have a very specific environment, this isn't applied to something like a house.
10 month baby
Homare's now 10 months!
In Ex. 22, some speakers would add 歳 to equate the mile stones of months for their child in the same way they as if they were to turn one or two.
日齢 exists, but it's the age of something born/birthed. So, it can work for humans or even eggs. We can even go smaller by considering 時（間）齢 (hour age), 分齢 (minute age), and 秒齢 (second age). These are certainly not used in the spoken language, but we can get around this.
14 day old eggs
His mother died when he was only five hours old/His mother died only five hours after he was born.
自然さ Note: 19b is more natural than 19a. Japanese does not like having to express age in things so small and would rather avoid it by using phrases such as "after" if possible.
In the case of people abroad who come home to give birth in Japan, a passport is needed for the zero year old child for returning overseas.
Phrase Note: Japanese has adopted ゼロ歳 and it is frequently used. You can also see 生後ゼロ歳ゼロヶ月.
Three second old bacteria → bacteria three seconds after splitting
Thirteen day old bacteria culture → Bacteria cultured for thirteen days
Phrasing Note: If English can avoid the phrase, then you can use that to help you understand how Japanese avoids "old".
This, however, wouldn't be a word that you would just use in conversation. The spoken language must have ways to go around this. With this in mind, consider the following. These examples show how Japanese takes close detail to the total semantic context.
Day old bread
Three day old sea urchin(s)
To boil five day old eggs since being laid.
Five day old baby
Curry one made two days ago
My wife's car is five years old/of five years. → It has been five years since my wife bought her car.