Muscle memory vs muscle wasting
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For people who work out, you will understand there are certain rules which if followed consistently lead to common results. For example, if you lift something heavy in a way which produces the desired combination of "mechanical tension", "metabolic stress", and "muscle damage", it signals to the body to release hormones required for muscle hypertrophy (muscle growth). 

Typically a person with average genetics can gain between 1 or 2 lbs of muscle a month. This means in a year a person can earn between 12 and 24lbs of muscle. The problem is to earn this muscle requires consistently keeping the body in an anabolic state (caloric surplus) as well as following a fitness routine which hits the requirements to signal muscle hypertrophy. Generally this fitness routine requires "progressive overload" which is to say the routine must increasingly get harder over time, so every 2 weeks the weight and volume must be increased or progress will stop or even potentially regress.

Muscle memory is a bit controversial. They say when you build muscle, say if you work out consistently 5 times a week for 1 year, and as a result earned 18lbs of muscle, if at the beginning of the next year you take a few months off, your muscles which you spent 12 months to build can be gone in 3 months of not working out. This muscle wasting happens under the "if you don't use it you lose it".  Muscle memory in studies has shown that once you have it then it is easier to get it back again.

Here is what is known about muscle hyperthropy 

- Humans cannot grow new muscle fibers. Existing fibers simply grow larger.
- When existing fibers grow larger, the cell nuclei amount grows. So while muscles have a fixed number of fibers set at birth the number of cell nuclei is not fixed.

So if a person lifts weights for a long time consistently then new cell growth takes place. These new cells make it easier for the muscles to grow in the future and we call it "muscle memory".  

The quote below explains:

> The muscle cells are the largest cells in the body with a volume thousands of times larger than most other body cells.[4] To support this large volume, the muscle cells are one of the very few in the mammalian body that contain several cell nuclei. Such multinucleated cells are called syncytia. Strength-training increases muscle mass and force mainly by changing the caliber of each fiber rather than increasing the number of fibers. 

>  Until recently it was believed that during muscle wasting (atrophy) muscle cells lost nuclei by a nuclear self-destruct mechanism called apoptosis, but recent observations using time laps in vivo imaging in mice do not support this model. Direct observation indicated that no nuclei are lost under such conditions,[5] and the apoptosis observed in the muscle tissue were demonstrated to occur only in other cell nuclei in the tissue, e.g. connective tissue and muscle stem cells called satellite cells. 

This could indicate that the earlier you begin lifting weights consistently the better it will be for you later on. This is because if you have more nuclei and these last a lot longer than muscle growth, then it makes regrowing easier. 

References
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1. [https://en.wikipedia.org/wiki/Muscle_memory_(strength_training)](https://en.wikipedia.org/wiki/Muscle_memory_(strength_training))

Schoenfeld, B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. The Journal of Strength & Conditioning Research, 24(10), 2857-2872.