the invested amount doesn't matter at all as investments are evaluated on ROI. Of course 20% profit from 10k$ is much more than from 10$, but if you have small money, invest small money. You can always use compound interest if you feel like swinging or trading the market. But take into account all the predictions we have for btc in 15-20 years - anywhere between 100k to 1m$ and beyond... so your 10 bucks can very well be worth 170-1700$ in that time and ask yourself if such an investment makes sense...
You are seriously thinking that investment will last forever so that you are really interested in compound interest for a long time? No investment is going to last forever that is why it is better to get only a little bit of money and play it safe. The amount doesnt matter as long as you can afford to lose it then it is fine to go with whatever amount