I want to complete this sentence:
Here we stand, around ten or fifteen years ago, when I started _____ about life.
The thinking is not necessarily focused on one particular topic. The closet word to think I found is meditate, but it too focuses on "religious or spiritual purposes or as a method of relaxation".
Metaphysics is, per Merriam-Webster,
a (1) : a division of philosophy that is concerned with the fundamental nature of reality and being and that includes ontology, cosmology, and often epistemology
(2) : ontology 2
b : abstract philosophical studies : a study of what is outside objective experience
which I think may be what you are trying to get at in your description of "thinking philosophically about life". If so, you can use the verb metaphysicize:
-ed/-ing/-s : to engage in metaphysical speculation
You wouldn't even need to use the last two words of your sentence; you could simply say
Here we stand, around ten or fifteen years ago, when I started metaphysicizing/to metaphysicize.
You don't need to say "about life" since the word on its own includes thinking philosophically about life, the universe, and everything.
On the other hand, if you don't so much mean that you started thinking deep thoughts about life but rather that you became more philosophical about life, in the sense
2 : characterized by the attitude of a philosopher; specifically : calm or unflinching in the face of trouble, defeat, or loss
then you may want to use a longer phrase like "to become more philosophical" or "to be more contemplative" or "to attain serenity".
One choice would be ruminating. See ruminate defined by Oxford dictionary as
Think deeply about something.
To ponder
to think or consider especially quietly, soberly, and deeply
Used in your sentence, it would probably work best as "pondering life," rather than "pondering about life."
I would use contemplate, and simply drop the word "about":
Here we stand, around ten or fifteen years ago, when I started contemplating life.
To philosophise:
to talk for a long time about subjects such as the meaning of life:
- Students, she complained, had nothing better to do than spend whole days philosophizing about the nature of truth.
(Cambridge Dictionary)
Also speculate conveys a more general and wider sense of thinking about something:
- to meditate on or ponder a subject : reflect
(M-W)
muse (MWD)
to become absorbed in thought; especially : to think about something carefully and thoroughly musing about what might have been
Here we stand, around ten or fifteen years ago, when I started thinking existentially about life.
If something is existential, it has to do with human existence. If you wrestle with big questions involving the meaning of life, you may be having an existential crisis. (vocabulary.com)
Philosophize, or philosophise, is probably the most suited to this task especially since it consists of the word and a suffix
http://www.thefreedictionary.com/philosophize
Here we stand, around ten or fifteen years ago, when I started philosophising about life.
contemplation:
deep reflective thought:
- he would retire to his room for study or contemplation.
I would consider saying you began to "wax philosophical."
Wax philosophical - To take on a quality or state of thinking or talking of deep subjects such as the meaning of life.
Is there any other way you can "wax" as you do when you "wax philosophical"?
Apart from ponder and contemplate, I would say you could use mull -
Think about (something) deeply and at length.
"She began to mull over the various notions to life"
It gives a feeling of serious contemplation which could be expected when one is thinking about topics like the one you have described.
