Nice tree. You have treated it well. As you have found most seedlings need to get very large before they will flower. You can prune this anytime you want but will most likely delay the eventual blooms.

most citrus are grafted onto the same rootstock. P. Trifoliate is one and there are certainly others. Most of this done because of size control, disease and pest resistance. Dwarfing is done with the rootstock so grafting onto this seedling wont necessarily accomplish that.
That doesnt mean you cant graft a different variety onto this tree.
Most small citrus are bud grafted where you take a bud from the desired variety and graft to the rootstock.
Basics of grafting is lining the cambium layer of the scion(top part) to the cambium of the rootstock.

Taking scion or bud wood from a tree in bloom will not hurt it at all.

You cam T-bud onto the smaller limbs. scions taker from mature trees will normally produce some fruit in about 3 years.

In response to mksmth, that seedling definitely looks big enough to have flowered by now. I'm not really knowledgeable when it comes to citrus, but the issue might be the roots. Being kept in a pot could be holding the tree back. I see some other plants growing down below in the same pot, this is probably not a good thing, root competition. generally not a good idea to be planting things around the base of a tree, not good for the tree.

Lemon trees also may not bloom if they are overwatered, the roots in that pot may not be getting adequate drainage.

It is often stated that a lemon tree grown from seed can take 6-10 years before it blooms, but there are people who have grown lemons from seed and had it bloom after only 1½ years.

I too am curious about grafting. I have purchased parafilm, and plan to attempt to graft onto what I believe is P. Trifoliate. Once the temperatures have warmed up a little more I shall attack. My game plan at this point was to attempt multiple bud grafts towards the base of the tree. The tree is over 6ft, and has a 1cm in diameter branch about a foot and a half above the ground. I was thinking I would bud graft around this area, and if the graft takes (stays green or the petiole easily falls off) I would then saw through eight tenths of the trunk above the graft. Therefore, leaving the branch towards the bottom alone, in the hopes it supports the graft. And if all goes well, eventually removing that last branch and remaining trunk.

Pecanman,
>You cam T-bud onto the smaller limbs. scions taker from mature trees will normally produce some fruit in about 3 years.
Would you ever T-bud onto the main trunk?

I like to T-bud citrus when the understock is pencil size to 3/8 inch in diameter. On larger plant i prefer to do a bark graft when the bark is slipping.

Do you split the bark at the top of the cut, or slightly down from it? Also, do you slice the tree and determine if the bark is slipping at that point? I've head pruning some of the tree, and providing lots of water are beneficial before grafting... but have yet to do any.....

thanks