well, the younger you are, the easier you absorb knowledge, but how fast you absorb knowledge has more to do with the learning method than what is being learned.
if a how-to guide is laid out well, then you will learn it faster than one that has been slapped together without thought. also, pictures in a guide, or an ability to put what you have learned to use right away will help keep someone learning what does what, and how things work within a language.
honestly, HTML is one of the easier languages out there (tricky to troubleshoot, though). if i was you, i would continue with learning Java or JavaScript, then PHP, then C and Python. progressively learning things which are SLIGHTLY more complex makes the learning go more smoothly. also, learning JavaScript and Java and PHP first (since they are related to HTML more, being more web-oriented), you will perhaps feel more familiar with the code, before delving into the more complex codes.
again, the speed at which you learn will vary, and while Python and C are more complex than JavaScript and PHP, those two are equally more complex than HTML...