Any computer, if using original parts, will get slower after a while. Things wear down over time even when they aren't being used, let alone used regularly. That being said, clean out the whole computer physically, wipe all the dust off, reapply thermal paste, decompress hardrive, make sure if HDD it isn't almost full. If all of these are some and it's still slow then yeah time for an upgrade.
If recommend, of you just want it for browsing and YouTube, getting a super cheap netbook rather than a full laptop.