1. He died at the age of 70.
2. He died at 70.
3. He died when he was 70 years old.
4. He died when he was 70.
5. He died aged 70.
(Are all grammatical?)
6. You will be taller than her in a couple of years.
6-1. You will be taller than her in two years.
(Are both the same?)
7. The next few years, you are growing in and out.
7-1. The next few years, you will grow in and out.
(Are both the same?)
#'s 1 though 5 are all used and are correct.
Both of the #6 sentences are incorrect. You will be taller than SHE in a couple of years (or in two years)
Neither of the 7 sentences make sense.
1. He died at the age of 70. - This sentence is grammatically correct and clear, stating that someone died when they were 70 years old.
2. He died at 70. - This sentence is also grammatically correct, but it is less explicit about the person's age at the time of their death. It can be inferred that the person died at the age of 70, but it is not explicitly stated.
3. He died when he was 70 years old. - This sentence is grammatically correct and explicitly states the person's age at the time of their death.
4. He died when he was 70. - This sentence is also grammatically correct, but like sentence 2, it does not explicitly state the person's age at the time of their death.
5. He died aged 70. - This sentence is also grammatically correct and implies the person's age at the time of their death.
Regarding your second set of questions:
6. You will be taller than her in a couple of years. - This sentence is grammatically correct and conveys that you will surpass her in height within a few years.
6-1. You will be taller than her in two years. - This sentence is essentially the same as the previous one but specifies the duration of two years instead of using the phrase "a couple of years." The meaning is unchanged, but the wording differs.
7. The next few years, you are growing in and out. - This sentence is grammatically incorrect. If you meant to convey that there will be periods of growth and decline over the next few years, a more accurate expression would be:
7-1. The next few years, you will grow in and out. - This sentence is grammatically correct and conveys the idea that there will be alternating periods of growth and decline in the future.