After playing the piano, you can learn any instrument to grade 8 within a year, unfortunately with the exception of string instruments. But if you play viola, flute and whatever else you play as well, you should have a very strong advantage.
At the age of 19, you are quite mature in playing, especially if you have been playing music for some time, and so understand music quite well. If you play viola, you will know quite a few string tecniques, ie. using the bow (although not with a cello bow hold etc. etc.).
If you also have some determination, ability to work hard and time, then you'll be able to succeed like mad.
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i took up flute and clarinet when i was 16 and got diploma and grade 8s when i was 18. is it going to be a similar case on cello?
No, not quite the same, it'll take you longer, but not quite as much longer as it would be for someone else (due to age, musical experience etc.).
If you do it and you don't get to professional standard, then hey, you haven't lost anything. The 'cello is an amazing instrument, everyone likes 'celli, and even if you're not a professional, you'll still be an amateur. Just make sure you can think of a witty comeback for the sentence 'I bet you wished you played the picallo!'
So do it. Even if it doesn't work, it'll still be a laugh.