Q: early 2009 macbook

Without knowing more about the 'early 2009' MacBook, or how your model is configured

between hard drive capacity, and RAM memory amounts, or free/unused drive capacity,

the answer is a definite maybe.

According to this article it shows technical specifications for El Capitan, plus vintage limits:

• OS X El Capitan - Technical Specifications

• OS X - How to Upgrade:

https://www.apple.com/osx/how-to-upgrade/

Some features of OS X 10.11.x won't work on older hardware. And any upgrade to a fully

new system should start with a complete backup of your files and additional TimeMachine

backup; some also prefer to make a complete clone of the entire earlier OS X and test it

to be sure you could revert to that if you found the computer didn't like the upgrade.

• How to identify MacBook models - Apple Support

If you have a larger hard disk drive and it shows to have plenty of additional unused free

capacity, it may be possible to create a partition in Disk Utility of some of that capacity,

to install the new OS X system as a test. That way the older one won't be compromised.

Usually older hardware configurations can run slower with a newer OS X installed. Some

users found an all-new installation may do OK; others not so much. There are two other

systems available but not from the App Store directly; instead as purchases from Apple

Store online. These are Lion 10.7 and Mountain Lion 10.8. Either sells for about $20.

from the Apple Store online, then a redemption code is used for App Store download.

And you may want to research their attributes (and Lion is kind of a dog, some said) if

your Mac can run El Capitan 10.11, it can also run Mountain Lion 10.8.5. You need not

buy either 10.7 or 10.8 to get El Capitan for free; but your old OS X applications may or

may not do OK in Mt Lion 10.8.5. Be sure to learn how to make an offline copy of the

newest OS X installers, since you can save time; everything is online now. An offline

compete backup to restore from clones and TimeMachine files can be very helpful later.

Examples of hardware replacement or upgrade:

http://eshop.macsales.com/shop/powerbook-ibook-macbook/

With the newer OS X, you'd have to consider the later applications (purchase from App

Store) if they were too old to run directly in the latest OS X. They may do OK in 10.8.5.

Mountain Lion at Apple Store:

http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion

The appearance of Mountain Lion 10.8(.5) is more like the older Snow Leopard; some of

those seeking to upgrade and install a new OS X in their Mac usually have or considering

new iPad, iPhone or other devices that may be dragging them forward to a new iOS and

a new OSX  (so they can share, sync, and so on) but if you don't need an upgrade for that

reason, you may have options. And the computer may do better with more RAM and a

larger capacity replacement hard disk drive. Also be sure to make several backups.

Good luck & happy computing!

Providing you have the available hard disk space yes you can. But I wouldn't do so unless you also have 6Gb of ram, El Capitan doesn't run well on older Macs with only 4Gb of ram. You should also consider replacing the spinning hard drive with an SSD for a real speed boost.