I started to pay close attention to BTC 3 weeks ago and its price was about $7000. And up to now its price was $12500, reaching it's peak. Is it appropriate to invest in bitcoins from now on？ 
Anyone that tells you it is a bad time is steering you wrong. If you can buy and hold your bitcoins, instead of spending them or getting antsy whenever you price moves, then you will be in marvelous shape as history and present day have/are teaching us time and time again. I was looking at the price chart of bitcoin since the beginning, at different intervals, days, months, years and how the price moved within those intervals, and I fail to see a bad time since the birth of bitcoin to invest.
From what I can see, if you put money into bitcoin and held if for 6-months, you never once could have made a bad investment into bitcoin. Basically, set yourself some ground-rules, like no spending coin for at least 6-months and you're ass guaranteed to make profit as you ever will be.
I'd love to be corrected if I'm wrong, but I'm almost positive, you could not have lost money on bitcoin if all you did was invest and hold for at least 6-months, at ANY given time since 2009. It's always appropriate to invest in bitcoin, invest consistently, and invest confidently.